"""
==================
Labelling subplots
==================

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

Simplest is putting the label inside the axes.  Note, here
we use `.pyplot.subplot_mosaic`, and use the subplot labels
as keys for the subplots, which is a nice convenience.  However,
the same method works with `.pyplot.subplots` or keys that are
different than what you want to label the subplot with.
"""

import matplotlib.pyplot as plt
import matplotlib.transforms as mtransforms

fig, axs = plt.subplot_mosaic([['a)', 'c)'], ['b)', 'c)'], ['d)', 'd)']],
                              layout='constrained')

for label, ax in axs.items():
    # label physical distance in and down:
    trans = mtransforms.ScaledTranslation(10/72, -5/72, fig.dpi_scale_trans)
    ax.text(0.0, 1.0, label, transform=ax.transAxes + trans,
            fontsize='medium', verticalalignment='top', fontfamily='serif',
            bbox=dict(facecolor='0.7', edgecolor='none', pad=3.0))

plt.show()

##############################################################################
# We may prefer the labels outside the axes, but still aligned
# with each other, in which case we use a slightly different transform:

fig, axs = plt.subplot_mosaic([['a)', 'c)'], ['b)', 'c)'], ['d)', 'd)']],
                              layout='constrained')

for label, ax in axs.items():
    # label physical distance to the left and up:
    trans = mtransforms.ScaledTranslation(-20/72, 7/72, fig.dpi_scale_trans)
    ax.text(0.0, 1.0, label, transform=ax.transAxes + trans,
            fontsize='medium', va='bottom', fontfamily='serif')

plt.show()

##############################################################################
# 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()

#############################################################################
#
# .. admonition:: References
#
#    The use of the following functions, methods, classes and modules is shown
#    in this example:
#
#    - `matplotlib.figure.Figure.subplot_mosaic` /
#      `matplotlib.pyplot.subplot_mosaic`
#    - `matplotlib.axes.Axes.set_title`
#    - `matplotlib.axes.Axes.text`
#    - `matplotlib.transforms.ScaledTranslation`