Ways to set a color's alpha value

Compare setting alpha by the alpha keyword argument and by one of the Matplotlib color formats. Often, the alpha keyword is the only tool needed to add transparency to a color. In some cases, the (matplotlib_color, alpha) color format provides an easy way to fine-tune the appearance of a Figure.

import matplotlib.pyplot as plt
import numpy as np

# Fixing random state for reproducibility.
np.random.seed(19680801)

fig, (ax1, ax2) = plt.subplots(ncols=2, figsize=(8, 4))

x_values = [n for n in range(20)]
y_values = np.random.randn(20)

facecolors = ['green' if y > 0 else 'red' for y in y_values]
edgecolors = facecolors

ax1.bar(x_values, y_values, color=facecolors, edgecolor=edgecolors, alpha=0.5)
ax1.set_title("Explicit 'alpha' keyword value\nshared by all bars and edges")


# Normalize y values to get distinct face alpha values.
abs_y = [abs(y) for y in y_values]
face_alphas = [n / max(abs_y) for n in abs_y]
edge_alphas = [1 - alpha for alpha in face_alphas]

colors_with_alphas = list(zip(facecolors, face_alphas))
edgecolors_with_alphas = list(zip(edgecolors, edge_alphas))

ax2.bar(x_values, y_values, color=colors_with_alphas,
        edgecolor=edgecolors_with_alphas)
ax2.set_title('Normalized alphas for\neach bar and each edge')

plt.show()

References

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

• matplotlib.axes.Axes.bar

• matplotlib.pyplot.subplots