Labelling subplots#

Labelling subplots is relatively straightforward, and varies, so Matplotlib does not have a general method for doing this.

We showcase two methods to position text at a given physical offset (in fontsize units or in points) away from a corner of the Axes: one using annotate, and one using ScaledTranslation.

For convenience, this example uses pyplot.subplot_mosaic and subplot labels as keys for the subplots. However, the approach also works with pyplot.subplots or keys that are different from what you want to label the subplot with.

import matplotlib.pyplot as plt

from matplotlib.transforms import ScaledTranslation

fig, axs = plt.subplot_mosaic([['a)', 'c)'], ['b)', 'c)'], ['d)', 'd)']],
                              layout='constrained')
for label, ax in axs.items():
    # Use Axes.annotate to put the label
    # - at the top left corner (axes fraction (0, 1)),
    # - offset half-a-fontsize right and half-a-fontsize down
    #   (offset fontsize (+0.5, -0.5)),
    # i.e. just inside the axes.
    ax.annotate(
        label,
        xy=(0, 1), xycoords='axes fraction',
        xytext=(+0.5, -0.5), textcoords='offset fontsize',
        fontsize='medium', verticalalignment='top', fontfamily='serif',
        bbox=dict(facecolor='0.7', edgecolor='none', pad=3.0))

fig, axs = plt.subplot_mosaic([['a)', 'c)'], ['b)', 'c)'], ['d)', 'd)']],
                              layout='constrained')
for label, ax in axs.items():
    # Use ScaledTranslation to put the label
    # - at the top left corner (axes fraction (0, 1)),
    # - offset 20 pixels left and 7 pixels up (offset points (-20, +7)),
    # i.e. just outside the axes.
    ax.text(
        0.0, 1.0, label, transform=(
            ax.transAxes + ScaledTranslation(-20/72, +7/72, fig.dpi_scale_trans)),
        fontsize='medium', va='bottom', fontfamily='serif')

If we want it aligned with the title, either incorporate in the title or use the loc keyword argument:

fig, axs = plt.subplot_mosaic([['a)', 'c)'], ['b)', 'c)'], ['d)', 'd)']],
                              layout='constrained')
for label, ax in axs.items():
    ax.set_title('Normal Title', fontstyle='italic')
    ax.set_title(label, fontfamily='serif', loc='left', fontsize='medium')

plt.show()

a), Normal Title, c), Normal Title, b), Normal Title, d), Normal Title

References

The use of the following functions, methods, classes and modules is shown in this example:

Total running time of the script: (0 minutes 2.223 seconds)

Download Jupyter notebook: label_subplots.ipynb

Download Python source code: label_subplots.py

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