Add figure letters to matplotlib axes

add_fig_letters.py

def add_fig_letters(axes, offset=.07, facecolor='#f9ecd2', **kwargs):
    """
    Add a figure letter to top-left corner for all axes
    
    Like is done in a publication figure, all axes are labeled with a
    letter so individual axes can be referred to from the text.

    Paramters
    ---------
    axes : list
        A list of matplotlib axes
    offset : number or tuple
        Fine tune the placement of the axes.
        If a single number given, will move in both x and y direction.
        If a tuple is give, first number is adjustment in x, 
        second number is adjustmen in y

    Example
    -------
    fig, axes = plt.subplots(4,4)
    add_fig_letters(axes)

    """
    if not hasattr(offset, '__len__'):
        offset = (offset, offset)
    
    assert len(offset) == 2, 'Offset must be a number or tuple'

    if not hasattr(axes, 'flat'): 
        np.array(axes)
    
    ### Add letters to plots
    import string
    
    try:
        axes = axes.flat 
    except:
        pass

    for i, (ax, letter) in enumerate(zip(axes, string.ascii_lowercase)):
        plt.sca(ax)

        # Add figure letter
        box_prop = dict(boxstyle='round',
                        facecolor=facecolor,
                        alpha=1,
                        linewidth=.5)
        
        plt.text(0+offset[0], 1-offset[1], f'{letter}', 
                 transform=ax.transAxes, fontfamily='monospace',
                 va='top', ha='left', 
                 bbox=box_prop, zorder=100_000, **kwargs)

This little function adds a quick letter label to each axes in a multi-axis matplotlib figure. I always need to do this when I create figures for publications, and this function makes the styles consistent for all my figures.

The function can be found in my toolbox.

import matplotlib.pyplot as plt
fig, axes = plt.subplots(3,4, figsize=[6,4], dpi=300, sharex=True, sharey=True)
add_fig_letters(axes)