Support callable for formatting of Sankey labels

The `format` parameter of `matplotlib.sankey.Sankey` can now accept callables.

This allows the use of an arbitrary function to label flows, for example allowing

the mapping of numbers to emoji.

.. plot::

import matplotlib.pyplot as plt

from matplotlib.sankey import Sankey

import math

# on Windows, this font may be necessary to display emojis

plt.rcParams['font.family'] = "Segoe UI Emoji"

def display_in_cats(values, min_cats, max_cats):

def display_in_cat_scale(value):

max_value = max(values, key=abs)

number_cats_to_show = max(min_cats, math.floor(abs(value) / max_value * max_cats))

return str(number_cats_to_show * '🐈')

return display_in_cat_scale

flows = [35, 15, 40, -20, -15, -5, -40, -10]

orientations = [-1, 1, 0, 1, 1, 1, -1, -1]

# Cats are good, we want a strictly positive number of them

min_cats = 1

# More than four cats might be too much for some people

max_cats = 4

cats_format = display_in_cats(flows, min_cats, max_cats)

sankey = Sankey(flows=flows, orientations=orientations, format=cats_format,

offset=.1, head_angle=180, shoulder=0, scale=.010)

diagrams = sankey.finish()

diagrams[0].texts[2].set_text('')

plt.title(f'Sankey flows measured in cats \n🐈 = {max(flows, key=abs) / max_cats}')

plt.show()