Individual alpha values in scatter plot
PythonMatplotlibAlphaScatter PlotPython Problem Overview
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.
Python Solutions
Solution 1 - Python
New solution with matplotlib >= 3.4
Since matplotlib 3.4, alpha supports an iterable of multiple values: https://matplotlib.org/stable/users/whats_new.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()
Solution 2 - Python
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()