Individual alpha values in scatter plot

Question

I’m wondering if it is possible to have individual alpha values for each point to be plotted using the scatter function of Matplotlib. I need to plot a set of points, each one with its alpha value.

For example, I have this code to plot some points

def plot_singularities(points_x, p, alpha_point, file_path):
    plt.figure()
    plt.scatter(points_x, points_y, alpha=alpha_point)
    plt.savefig(file_path + '.png', dpi=100)
    plt.close()

All my points_x, points_y and alpha_point have n values. However, I can’t assign an array to the alpha parameter in scatter(). How can I have a different alpha value for each point? I can loop and plot point by point with each specific alpha value, but this doesn’t seem like a good approach.

Asked By: pceccon

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Source

Answer 1

New solution with matplotlib >= 3.4

Since matplotlib 3.4, alpha supports an iterable of multiple values:
https://matplotlib.org/stable/users/prev_whats_new/whats_new_3.4.0.html#transparency-alpha-can-be-set-as-an-array-in-collections

import numpy as np
import matplotlib.pylab as plt

x = np.arange(10)
y = np.arange(10)

alphas = np.linspace(0.1, 1, 10)

plt.scatter(x, y, alpha=alphas)
plt.show()

Old solution for matplotlib < 3.4

tcaswell’s suggestion is correct, you can do it like this:

import numpy as np
import matplotlib.pylab as plt

x = np.arange(10)
y = np.arange(10)

alphas = np.linspace(0.1, 1, 10)
rgba_colors = np.zeros((10,4))
# for red the first column needs to be one
rgba_colors[:,0] = 1.0
# the fourth column needs to be your alphas
rgba_colors[:, 3] = alphas

plt.scatter(x, y, color=rgba_colors)
plt.show()

Answered By: MaxNoe

Answer 2

You can use the color argument and a colormap with alpha.
cmap linearly increases the alpha value from 0 to 1.

import numpy as np
import matplotlib.pylab as plt
from matplotlib import colors

c='C0'

xs = np.arange(10)

fig, ax = plt.subplots(1, 1)
cmap = colors.LinearSegmentedColormap.from_list(
        'incr_alpha', [(0, (*colors.to_rgb(c),0)), (1, c)])
ax.scatter(xs, xs, c=xs, cmap=cmap, ec=None, s=10**2)

plt.show()

Answered By: Markus Dutschke

Individual alpha values in scatter plot

Question:

Answers:

New solution with matplotlib >= 3.4

Old solution for matplotlib < 3.4