Matplotlib Smooth Grey-Red Color Bar

Question

I am struggling to create the color bar that I want in matplotlib. Ideally it would look like more like this (lows are greys, highs are bright red:

However I am getting the following:

from matplotlib import cm
from matplotlib.colors import ListedColormap, LinearSegmentedColormap

colors_top = cm.get_cmap('Greys_r', 128)
colors_bot = cm.get_cmap('Reds', 128)
cmap_grey_red = ListedColormap(
    np.vstack((
        colors_top(np.linspace(0, 1, 128)),
        colors_bot(np.linspace(0, 1, 128))
    )), name='GreyRed'
)

How can I correct this color bar?

This appears related: https://stackoverflow.com/a/25996834/2864250 — dubbbdan, Jul 21 '22 at 17:39
@dubbbdan yes somewhat related. `LinearSegmentedColormap.from_list("GreyRed",['grey', "r"])` is closer, still not as nice as I would like it though... thanks for the pointer! — SumNeuron, Jul 21 '22 at 18:29

score 2 · Answer 1 · answered Jul 21 '22 at 19:05

Whatever approach you choose, you will still have to map the colors to whatever scale the color bar is displaying using the norm argument. Here is what I could quickly come up with:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.colors

x,y,c = zip(*np.random.rand(30,3)*4-2)

cvals  = [-2., 0,  2]
colors = ["dimgray","salmon","red"]

norm=plt.Normalize(min(cvals),max(cvals))
tuples = list(zip(map(norm,cvals), colors))
cmap = matplotlib.colors.LinearSegmentedColormap.from_list("", tuples)

plt.scatter(x,y,c=c, cmap=cmap, norm=norm)
plt.colorbar()
plt.show()

The color ramp you provided is diverging (e.g., white in the middle), If you want a smoother transition, you could just develop a color map with three colors.

Matplotlib Smooth Grey-Red Color Bar

1 Answers1