Source code for pybamm.plotting.quick_plot

#
# Class for quick plotting of variables from models
#
import os
import numpy as np
import pybamm
from collections import defaultdict
from pybamm.util import import_optional_dependency


class LoopList(list):
    """A list which loops over itself when accessing an
    index so that it never runs out"""

    def __getitem__(self, i):
        # implement looping by calling "(i) modulo (length of list)"
        return super().__getitem__(i % len(self))


def ax_min(data):
    """Calculate appropriate minimum axis value for plotting"""
    data_min = np.nanmin(data)
    data_max = np.nanmax(data)
    return data_max - 1.05 * (data_max - data_min)


def ax_max(data):
    """Calculate appropriate maximum axis value for plotting"""
    data_min = np.nanmin(data)
    data_max = np.nanmax(data)
    return data_min + 1.05 * (data_max - data_min)


def split_long_string(title, max_words=None):
    """Get title in a nice format"""
    max_words = max_words or pybamm.settings.max_words_in_line
    words = title.split()
    # Don't split if fits on one line, don't split just for units
    if len(words) <= max_words or words[max_words].startswith("["):
        return title
    else:
        first_line = (" ").join(words[:max_words])
        second_line = (" ").join(words[max_words:])
        return first_line + "\n" + second_line


def close_plots():
    """Close all open figures"""
    plt = import_optional_dependency("matplotlib.pyplot")

    plt.close("all")


[docs] class QuickPlot: """ Generates a quick plot of a subset of key outputs of the model so that the model outputs can be easily assessed. Parameters ---------- solutions: (iter of) :class:`pybamm.Solution` or :class:`pybamm.Simulation` The numerical solution(s) for the model(s), or the simulation object(s) containing the solution(s). output_variables : list of str, optional List of variables to plot labels : list of str, optional Labels for the different models. Defaults to model names colors : list of str, optional The colors to loop over when plotting. Defaults to None, in which case the default color loop defined by matplotlib style sheet or rcParams is used. linestyles : list of str, optional The linestyles to loop over when plotting. Defaults to ["-", ":", "--", "-."] shading : str, optional The shading to use for 2D plots. Defaults to "auto". figsize : tuple of floats, optional The size of the figure to make n_rows : int, optional The number of rows to use. If None (default), floor(n // sqrt(n)) is used where n = len(output_variables) so that the plot is as square as possible time_unit : str, optional Format for the time output ("hours", "minutes", or "seconds") spatial_unit : str, optional Format for the spatial axes ("m", "mm", or "um") variable_limits : str or dict of str, optional How to set the axis limits (for 0D or 1D variables) or colorbar limits (for 2D variables). Options are: - "fixed" (default): keep all axes fixes so that all data is visible - "tight": make axes tight to plot at each time - dictionary: fine-grain control for each variable, can be either "fixed" or \ "tight" or a specific tuple (lower, upper). """ def __init__( self, solutions, output_variables=None, labels=None, colors=None, linestyles=None, shading="auto", figsize=None, n_rows=None, time_unit=None, spatial_unit="um", variable_limits="fixed", ): solutions = self.preprocess_solutions(solutions) models = [solution.all_models[0] for solution in solutions] # Set labels if labels is None: self.labels = [model.name for model in models] else: if len(labels) != len(models): raise ValueError( f"labels '{labels}' have different length to models '{[model.name for model in models]}'" ) self.labels = labels # Set colors, linestyles, figsize, axis limits # call LoopList to make sure list index never runs out if colors is None: self.colors = LoopList(colors or ["r", "b", "k", "g", "m", "c"]) else: self.colors = LoopList(colors) self.linestyles = LoopList(linestyles or ["-", ":", "--", "-."]) self.shading = shading # Default output variables for lead-acid and lithium-ion if output_variables is None: output_variables = models[0].default_quick_plot_variables # raise error if still None if output_variables is None: raise ValueError( f"No default output variables provided for {models[0].name}" ) # check variables have been provided after any serialisation if any(len(m.variables) == 0 for m in models): raise AttributeError("No variables to plot") self.n_rows = n_rows or int( len(output_variables) // np.sqrt(len(output_variables)) ) self.n_cols = int(np.ceil(len(output_variables) / self.n_rows)) figwidth_default = min(15, 4 * self.n_cols) figheight_default = min(8, 1 + 3 * self.n_rows) self.figsize = figsize or (figwidth_default, figheight_default) # Spatial scales (default to 1 if information not in model) if spatial_unit == "m": self.spatial_factor = 1 self.spatial_unit = "m" elif spatial_unit == "mm": self.spatial_factor = 1e3 self.spatial_unit = "mm" elif spatial_unit == "um": # micrometers self.spatial_factor = 1e6 self.spatial_unit = r"$\mu$m" else: raise ValueError(f"spatial unit '{spatial_unit}' not recognized") # Time parameters self.ts_seconds = [solution.t for solution in solutions] min_t = np.min([t[0] for t in self.ts_seconds]) max_t = np.max([t[-1] for t in self.ts_seconds]) # Set timescale if time_unit is None: # defaults depend on how long the simulation is if max_t >= 3600: time_scaling_factor = 3600 # time in hours self.time_unit = "h" else: time_scaling_factor = 1 # time in seconds self.time_unit = "s" elif time_unit == "seconds": time_scaling_factor = 1 self.time_unit = "s" elif time_unit == "minutes": time_scaling_factor = 60 self.time_unit = "min" elif time_unit == "hours": time_scaling_factor = 3600 self.time_unit = "h" else: raise ValueError(f"time unit '{time_unit}' not recognized") self.time_scaling_factor = time_scaling_factor self.min_t = min_t / time_scaling_factor self.max_t = max_t / time_scaling_factor # Prepare dictionary of variables # output_variables is a list of strings or lists, e.g. # ["var 1", ["variable 2", "var 3"]] output_variable_tuples = [] self.variable_limits = {} for variable_list in output_variables: # Make sure we always have a list of lists of variables, e.g. # [["var 1"], ["variable 2", "var 3"]] if isinstance(variable_list, str): variable_list = [variable_list] # Store the key as a tuple variable_tuple = tuple(variable_list) output_variable_tuples.append(variable_tuple) # axis limits if variable_limits in ["fixed", "tight"]: self.variable_limits[variable_tuple] = variable_limits else: # If there is only one variable, extract it if len(variable_tuple) == 1: variable = variable_tuple[0] else: variable = variable_tuple try: self.variable_limits[variable_tuple] = variable_limits[variable] except KeyError: # if variable_tuple is not provided, default to "fixed" self.variable_limits[variable_tuple] = "fixed" except TypeError as error: raise TypeError( "variable_limits must be 'fixed', 'tight', or a dict" ) from error self.set_output_variables(output_variable_tuples, solutions) self.reset_axis() @staticmethod def preprocess_solutions(solutions): input_solutions = QuickPlot.check_input_validity(solutions) processed_solutions = [] for sim_or_sol in input_solutions: if isinstance(sim_or_sol, pybamm.Simulation): # 'sim_or_sol' is actually a 'Simulation' object here, so it has a # 'Solution' attribute processed_solutions.append(sim_or_sol.solution) elif isinstance(sim_or_sol, pybamm.Solution): processed_solutions.append(sim_or_sol) return processed_solutions @staticmethod def check_input_validity(input_solutions): if not isinstance(input_solutions, (pybamm.Solution, pybamm.Simulation, list)): raise TypeError( "Solutions must be 'pybamm.Solution' or 'pybamm.Simulation' or list" ) elif not isinstance(input_solutions, list): input_solutions = [input_solutions] else: if not input_solutions: raise TypeError("QuickPlot requires at least 1 solution or simulation.") return input_solutions def set_output_variables(self, output_variables, solutions): # Set up output variables self.variables = {} self.spatial_variable_dict = {} self.first_spatial_variable = {} self.second_spatial_variable = {} self.x_first_and_y_second = {} self.is_y_z = {} # Calculate subplot positions based on number of variables supplied self.subplot_positions = {} for k, variable_tuple in enumerate(output_variables): # Prepare list of variables variables = [None] * len(solutions) # process each variable in variable_list for each model for i, solution in enumerate(solutions): # variables lists of lists, so variables[i] is a list variables[i] = [] for var in variable_tuple: sol = solution[var] # Check variable isn't all-nan if np.all(np.isnan(sol.entries)): raise ValueError(f"All-NaN variable '{var}' provided") # If ok, add to the list of solutions else: variables[i].append(sol) # Make sure variables have the same dimensions and domain # just use the first solution to check this first_solution = variables[0] first_variable = first_solution[0] domain = first_variable.domain # check all other solutions against the first solution for idx, variable in enumerate(first_solution): if variable.domain != domain: raise ValueError( "Mismatching variable domains. " f"'{variable_tuple[0]}' has domain '{domain}', but '{variable_tuple[idx]}' has domain '{variable.domain}'" ) self.spatial_variable_dict[variable_tuple] = {} # Set the x variable (i.e. "x" or "r" for any one-dimensional variables) if first_variable.dimensions == 1: (spatial_var_name, spatial_var_value) = self.get_spatial_var( variable_tuple, first_variable, "first" ) self.spatial_variable_dict[variable_tuple] = { spatial_var_name: spatial_var_value } self.first_spatial_variable[variable_tuple] = ( spatial_var_value * self.spatial_factor ) elif first_variable.dimensions == 2: # Don't allow 2D variables if there are multiple solutions if len(variables) > 1: raise NotImplementedError( "Cannot plot 2D variables when comparing multiple solutions, " f"but '{variable_tuple[0]}' is 2D" ) # But do allow if just a single solution else: # Add both spatial variables to the variable_tuples ( first_spatial_var_name, first_spatial_var_value, ) = self.get_spatial_var(variable_tuple, first_variable, "first") ( second_spatial_var_name, second_spatial_var_value, ) = self.get_spatial_var(variable_tuple, first_variable, "second") self.spatial_variable_dict[variable_tuple] = { first_spatial_var_name: first_spatial_var_value, second_spatial_var_name: second_spatial_var_value, } self.first_spatial_variable[variable_tuple] = ( first_spatial_var_value * self.spatial_factor ) self.second_spatial_variable[variable_tuple] = ( second_spatial_var_value * self.spatial_factor ) # different order based on whether the domains # are x-r, x-z or y-z, etc if ( first_spatial_var_name in ("r", "R") and second_spatial_var_name == "x" ): self.x_first_and_y_second[variable_tuple] = False self.is_y_z[variable_tuple] = False elif ( first_spatial_var_name == "y" and second_spatial_var_name == "z" ): self.x_first_and_y_second[variable_tuple] = True self.is_y_z[variable_tuple] = True else: self.x_first_and_y_second[variable_tuple] = True self.is_y_z[variable_tuple] = False # Store variables and subplot position self.variables[variable_tuple] = variables self.subplot_positions[variable_tuple] = (self.n_rows, self.n_cols, k + 1)
[docs] def get_spatial_var(self, key, variable, dimension): """Return the appropriate spatial variable(s)""" # Extract name and value # Special case for current collector, which is 2D but in a weird way (both # first and second variables are in the same domain, not auxiliary domain) if dimension == "first": spatial_var_name = variable.first_dimension spatial_var_value = variable.first_dim_pts domain = variable.domain[0] elif dimension == "second": spatial_var_name = variable.second_dimension spatial_var_value = variable.second_dim_pts if variable.domain[0] == "current collector": domain = "current collector" else: domain = variable.domains["secondary"][0] if domain == "current collector": domain += f" {spatial_var_name}" return spatial_var_name, spatial_var_value
[docs] def reset_axis(self): """ Reset the axis limits to the default values. These are calculated to fit around the minimum and maximum values of all the variables in each subplot """ self.axis_limits = {} for key, variable_lists in self.variables.items(): if variable_lists[0][0].dimensions == 0: x_min = self.min_t x_max = self.max_t elif variable_lists[0][0].dimensions == 1: x_min = self.first_spatial_variable[key][0] x_max = self.first_spatial_variable[key][-1] elif variable_lists[0][0].dimensions == 2: # different order based on whether the domains are x-r, x-z or y-z, etc if self.x_first_and_y_second[key] is False: x_min = self.second_spatial_variable[key][0] x_max = self.second_spatial_variable[key][-1] y_min = self.first_spatial_variable[key][0] y_max = self.first_spatial_variable[key][-1] else: x_min = self.first_spatial_variable[key][0] x_max = self.first_spatial_variable[key][-1] y_min = self.second_spatial_variable[key][0] y_max = self.second_spatial_variable[key][-1] # Create axis for contour plot self.axis_limits[key] = [x_min, x_max, y_min, y_max] # Get min and max variable values if self.variable_limits[key] == "fixed": # fixed variable limits: calculate "globlal" min and max spatial_vars = self.spatial_variable_dict[key] var_min = np.min( [ ax_min(var(self.ts_seconds[i], **spatial_vars, warn=False)) for i, variable_list in enumerate(variable_lists) for var in variable_list ] ) var_max = np.max( [ ax_max(var(self.ts_seconds[i], **spatial_vars, warn=False)) for i, variable_list in enumerate(variable_lists) for var in variable_list ] ) if np.isnan(var_min) or np.isnan(var_max): raise ValueError( "The variable limits are set to 'fixed' but the min and max " "values are NaN" ) if var_min == var_max: var_min -= 1 var_max += 1 elif self.variable_limits[key] == "tight": # tight variable limits: axes will adjust each time var_min, var_max = None, None else: # user-specified axis limits var_min, var_max = self.variable_limits[key] if variable_lists[0][0].dimensions in [0, 1]: self.axis_limits[key] = [x_min, x_max, var_min, var_max] else: self.variable_limits[key] = (var_min, var_max) if ( var_min is not None and var_max is not None and (np.isnan(var_min) or np.isnan(var_max)) ): # pragma: no cover raise ValueError(f"Axis limits cannot be NaN for variables '{key}'")
[docs] def plot(self, t, dynamic=False): """Produces a quick plot with the internal states at time t. Parameters ---------- t : float Dimensional time (in 'time_units') at which to plot. dynamic : bool, optional If True, creates a dynamic plot with a slider. """ plt = import_optional_dependency("matplotlib.pyplot") gridspec = import_optional_dependency("matplotlib.gridspec") cm = import_optional_dependency("matplotlib", "cm") colors = import_optional_dependency("matplotlib", "colors") t_in_seconds = t * self.time_scaling_factor self.fig = plt.figure(figsize=self.figsize) self.gridspec = gridspec.GridSpec(self.n_rows, self.n_cols) self.plots = {} self.time_lines = {} self.colorbars = {} self.axes = QuickPlotAxes() # initialize empty handles, to be created only if the appropriate plots are made solution_handles = [] for k, (key, variable_lists) in enumerate(self.variables.items()): ax = self.fig.add_subplot(self.gridspec[k]) self.axes.add(key, ax) x_min, x_max, y_min, y_max = self.axis_limits[key] ax.set_xlim(x_min, x_max) if y_min is not None and y_max is not None: ax.set_ylim(y_min, y_max) ax.xaxis.set_major_locator(plt.MaxNLocator(3)) self.plots[key] = defaultdict(dict) variable_handles = [] # Set labels for the first subplot only (avoid repetition) if variable_lists[0][0].dimensions == 0: # 0D plot: plot as a function of time, indicating time t with a line ax.set_xlabel(f"Time [{self.time_unit}]") for i, variable_list in enumerate(variable_lists): for j, variable in enumerate(variable_list): if len(variable_list) == 1: # single variable -> use linestyle to differentiate model linestyle = self.linestyles[i] else: # multiple variables -> use linestyle to differentiate # variables (color differentiates models) linestyle = self.linestyles[j] full_t = self.ts_seconds[i] (self.plots[key][i][j],) = ax.plot( full_t / self.time_scaling_factor, variable(full_t, warn=False), color=self.colors[i], linestyle=linestyle, ) variable_handles.append(self.plots[key][0][j]) solution_handles.append(self.plots[key][i][0]) y_min, y_max = ax.get_ylim() ax.set_ylim(y_min, y_max) (self.time_lines[key],) = ax.plot( [ t_in_seconds / self.time_scaling_factor, t_in_seconds / self.time_scaling_factor, ], [y_min, y_max], "k--", lw=1.5, ) elif variable_lists[0][0].dimensions == 1: # 1D plot: plot as a function of x at time t # Read dictionary of spatial variables spatial_vars = self.spatial_variable_dict[key] spatial_var_name = next(iter(spatial_vars.keys())) ax.set_xlabel( f"{spatial_var_name} [{self.spatial_unit}]", ) for i, variable_list in enumerate(variable_lists): for j, variable in enumerate(variable_list): if len(variable_list) == 1: # single variable -> use linestyle to differentiate model linestyle = self.linestyles[i] else: # multiple variables -> use linestyle to differentiate # variables (color differentiates models) linestyle = self.linestyles[j] (self.plots[key][i][j],) = ax.plot( self.first_spatial_variable[key], variable(t_in_seconds, **spatial_vars, warn=False), color=self.colors[i], linestyle=linestyle, zorder=10, ) variable_handles.append(self.plots[key][0][j]) solution_handles.append(self.plots[key][i][0]) # add lines for boundaries between subdomains for boundary in variable_lists[0][0].internal_boundaries: boundary_scaled = boundary * self.spatial_factor ax.axvline(boundary_scaled, color="0.5", lw=1, zorder=0) elif variable_lists[0][0].dimensions == 2: # Read dictionary of spatial variables spatial_vars = self.spatial_variable_dict[key] # there can only be one entry in the variable list variable = variable_lists[0][0] # different order based on whether the domains are x-r, x-z or y-z, etc if self.x_first_and_y_second[key] is False: x_name = list(spatial_vars.keys())[1][0] y_name = next(iter(spatial_vars.keys()))[0] x = self.second_spatial_variable[key] y = self.first_spatial_variable[key] var = variable(t_in_seconds, **spatial_vars, warn=False) else: x_name = next(iter(spatial_vars.keys()))[0] y_name = list(spatial_vars.keys())[1][0] x = self.first_spatial_variable[key] y = self.second_spatial_variable[key] var = variable(t_in_seconds, **spatial_vars, warn=False).T ax.set_xlabel(f"{x_name} [{self.spatial_unit}]") ax.set_ylabel(f"{y_name} [{self.spatial_unit}]") vmin, vmax = self.variable_limits[key] # store the plot and the var data (for testing) as cant access # z data from QuadMesh or QuadContourSet object if self.is_y_z[key] is True: self.plots[key][0][0] = ax.pcolormesh( x, y, var, vmin=vmin, vmax=vmax, shading=self.shading, ) else: self.plots[key][0][0] = ax.contourf( x, y, var, levels=100, vmin=vmin, vmax=vmax ) self.plots[key][0][1] = var if vmin is None and vmax is None: vmin = ax_min(var) vmax = ax_max(var) self.colorbars[key] = self.fig.colorbar( cm.ScalarMappable(colors.Normalize(vmin=vmin, vmax=vmax)), ax=ax, ) # Set either y label or legend entries if len(key) == 1: title = split_long_string(key[0]) ax.set_title(title, fontsize="medium") else: ax.legend( variable_handles, [split_long_string(s, 6) for s in key], bbox_to_anchor=(0.5, 1), loc="lower center", ) # Set global legend if len(self.labels) > 1: fig_legend = self.fig.legend( solution_handles, self.labels, loc="lower right" ) # Get the position of the top of the legend in relative figure units # There may be a better way ... try: legend_top_inches = fig_legend.get_window_extent( renderer=self.fig.canvas.get_renderer() ).get_points()[1, 1] fig_height_inches = (self.fig.get_size_inches() * self.fig.dpi)[1] legend_top = legend_top_inches / fig_height_inches except AttributeError: # pragma: no cover # When testing the examples we set the matplotlib backend to "Template" # which means that the above code doesn't work. Since this is just for # that particular test we can just skip it legend_top = 0 else: legend_top = 0 # Fix layout if dynamic: slider_top = 0.05 else: slider_top = 0 bottom = max(legend_top, slider_top) self.gridspec.tight_layout(self.fig, rect=[0, bottom, 1, 1])
[docs] def dynamic_plot(self, show_plot=True, step=None): """ Generate a dynamic plot with a slider to control the time. Parameters ---------- step : float, optional For notebook mode, size of steps to allow in the slider. Defaults to 1/100th of the total time. show_plot : bool, optional Whether to show the plots. Default is True. Set to False if you want to only display the plot after plt.show() has been called. """ if pybamm.is_notebook(): # pragma: no cover import ipywidgets as widgets step = step or self.max_t / 100 widgets.interact( lambda t: self.plot(t, dynamic=False), t=widgets.FloatSlider( min=self.min_t, max=self.max_t, step=step, value=self.min_t ), continuous_update=False, ) else: plt = import_optional_dependency("matplotlib.pyplot") Slider = import_optional_dependency("matplotlib.widgets", "Slider") # create an initial plot at time self.min_t self.plot(self.min_t, dynamic=True) axcolor = "lightgoldenrodyellow" ax_slider = plt.axes([0.315, 0.02, 0.37, 0.03], facecolor=axcolor) self.slider = Slider( ax_slider, f"Time [{self.time_unit}]", self.min_t, self.max_t, valinit=self.min_t, color="#1f77b4", ) self.slider.on_changed(self.slider_update) if show_plot: # pragma: no cover plt.show()
[docs] def slider_update(self, t): """ Update the plot in self.plot() with values at new time """ from matplotlib import cm, colors time_in_seconds = t * self.time_scaling_factor for k, (key, plot) in enumerate(self.plots.items()): ax = self.axes[k] if self.variables[key][0][0].dimensions == 0: self.time_lines[key].set_xdata([t]) elif self.variables[key][0][0].dimensions == 1: var_min = np.inf var_max = -np.inf for i, variable_lists in enumerate(self.variables[key]): for j, variable in enumerate(variable_lists): var = variable( time_in_seconds, **self.spatial_variable_dict[key], warn=False, ) plot[i][j].set_ydata(var) var_min = min(var_min, ax_min(var)) var_max = max(var_max, ax_max(var)) # update boundaries between subdomains y_min, y_max = self.axis_limits[key][2:] if y_min is None and y_max is None: ax.set_ylim(var_min, var_max) elif self.variables[key][0][0].dimensions == 2: # 2D plot: plot as a function of x and y at time t # Read dictionary of spatial variables spatial_vars = self.spatial_variable_dict[key] # there can only be one entry in the variable list variable = self.variables[key][0][0] vmin, vmax = self.variable_limits[key] if self.x_first_and_y_second[key] is False: x = self.second_spatial_variable[key] y = self.first_spatial_variable[key] var = variable(time_in_seconds, **spatial_vars, warn=False) else: x = self.first_spatial_variable[key] y = self.second_spatial_variable[key] var = variable(time_in_seconds, **spatial_vars, warn=False).T # store the plot and the var data (for testing) as cant access # z data from QuadMesh or QuadContourSet object if self.is_y_z[key] is True: self.plots[key][0][0] = ax.pcolormesh( x, y, var, vmin=vmin, vmax=vmax, shading=self.shading, ) else: self.plots[key][0][0] = ax.contourf( x, y, var, levels=100, vmin=vmin, vmax=vmax ) self.plots[key][0][1] = var if (vmin, vmax) == (None, None): vmin = ax_min(var) vmax = ax_max(var) cb = self.colorbars[key] cb.update_normal( cm.ScalarMappable(colors.Normalize(vmin=vmin, vmax=vmax)) ) self.fig.canvas.draw_idle()
[docs] def create_gif(self, number_of_images=80, duration=0.1, output_filename="plot.gif"): """ Generates x plots over a time span of max_t - min_t and compiles them to create a GIF. Parameters ---------- number_of_images : int, optional Number of images/plots to be compiled for a GIF. duration : float, optional Duration of visibility of a single image/plot in the created GIF. output_filename : str, optional Name of the generated GIF file. """ imageio = import_optional_dependency("imageio.v2") plt = import_optional_dependency("matplotlib.pyplot") # time stamps at which the images/plots will be created time_array = np.linspace(self.min_t, self.max_t, num=number_of_images) images = [] # create images/plots stub_name = output_filename.split(".")[0] for val in time_array: self.plot(val) temp_name = f"{stub_name}{val}.png" images.append(temp_name) self.fig.savefig(temp_name, dpi=300) plt.close() # compile the images/plots to create a GIF with imageio.get_writer(output_filename, mode="I", duration=duration) as writer: for image in images: writer.append_data(imageio.imread(image)) # remove the generated images for image in images: os.remove(image)
[docs] class QuickPlotAxes: """ Class to store axes for the QuickPlot """ def __init__(self): self._by_variable = {} self._axes = []
[docs] def add(self, keys, axis): """ Add axis Parameters ---------- keys : iter Iterable of keys of variables being plotted on the axis axis : matplotlib Axis object The axis object """ self._axes.append(axis) for k in keys: self._by_variable[k] = axis
def __getitem__(self, index): """ Get axis by index """ return self._axes[index]
[docs] def by_variable(self, key): """ Get axis by variable name """ return self._by_variable[key]