How to fill a polygon with a custom hatch in Matplotlib?

To fill a polygon with a custom hatch in matplotlib, we can override the matplotlib.hatch.Shapes class.

Steps

Set the figure size and adjust the padding between and around the subplots.
Make a hatch shape using polygon class and get the path.
Override the custom hatch shape class, with shape_vertices, shape_codes, etc.
Create a new figure or activate an existing figure.
Add an axes to the figure as part of a subplot arrangement.
Add an axes patch of shape polygon.
Set the hatching pattern.
To display the figure, use show() method.

Example

import matplotlib.hatch
import matplotlib.pyplot as plt
from matplotlib.patches import Polygon

plt.rcParams["figure.figsize"] = [7.50, 3.50]
plt.rcParams["figure.autolayout"] = True

my_hatch_shape = Polygon(
   [[0., 0.4], [-0.3, 0.1], [0.1, 0.2]],
   closed=True, fill=False).get_path()

class MyCustomHatch(matplotlib.hatch.Shapes):
   filled = True
   size = 1.0
   path = my_hatch_shape

   def __init__(self, hatch, density):
      self.num_rows = (hatch.count('c')) * density
      self.shape_vertices = self.path.vertices
      self.shape_codes = self.path.codes
      matplotlib.hatch.Shapes.__init__(self, hatch, density)

matplotlib.hatch._hatch_types.append(MyCustomHatch)

fig = plt.figure()
ax = fig.add_subplot(111)

pg = ax.add_patch(Polygon(
   [[0.2, 0.2], [0.2, 0.8], [0.8, 0.1]],
   closed=True, fill=False))

pg.set_hatch('c')

plt.show()