get aspect ratio of axes

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From the docs, the aspect ratio is the ratio of data-to-display scaling units in the x- and y-axes. i.e., if there are 10 data units per display in the y-direction and 1 data unit per display unit in the x-direction, the ratio would be 1/10. The circle would be 10 times wider than it is tall. This corresponds to the statement that an aspect of num does the following:

a circle will be stretched such that the height is num times the width. aspect=1 is the same as aspect=’equal’.

Based on the same original piece of code that you looked at (matplotlib.axes._base.adjust_aspect, starting around line 1405), I think we can come up with a simplified formula as long as you only need this ratio for linear Cartesian axes. Things get complicated with polar and logarithmic axes, so I will ignore them.

To reiterate the formula:

(x_data_unit / x_display_unit) / (y_data_unit / y_display_unit)

This happens to be the same as

(y_display_unit / x_display_unit) / (y_data_unit / x_data_unit)

This last formulation is just the ratio of the display sizes in the two directions divided by the ratio of the x and y limits. Note that ax.get_data_ratio does NOT apply here because that returns the results for the actual data bounds, not the axis limits at all:

from operator import sub
def get_aspect(ax):
    # Total figure size
    figW, figH = ax.get_figure().get_size_inches()
    # Axis size on figure
    _, _, w, h = ax.get_position().bounds
    # Ratio of display units
    disp_ratio = (figH * h) / (figW * w)
    # Ratio of data units
    # Negative over negative because of the order of subtraction
    data_ratio = sub(*ax.get_ylim()) / sub(*ax.get_xlim())

    return disp_ratio / data_ratio

Now let’s test it:

from matplotlib import pyplot as plt
fig, ax = plt.subplots()
ax.set_aspect('equal')
print('{} == {}'.format(ax.get_aspect(), get_aspect(ax)))
ax.set_aspect(10)
print('{} == {}'.format(ax.get_aspect(), get_aspect(ax)))

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