Splitting the range of colorbar in Python

Question

In the following code, I am representing the data through array X on different colors. But I want to limit the number of colors to 6 (and not 8 as shown in the output) with each range being 100. Is there a way to do it?

import matplotlib as mpl
import matplotlib.pyplot as plt
from matplotlib.patches import Rectangle
import numpy as np
from matplotlib.colors import Normalize
from matplotlib import cm
import math
from  numpy import nan

fig,ax = plt.subplots(1)
n=3


N=2*n*(n-1)


J=[[3, 6, 7, 8, 9, 10, 11]]

J[0].sort()
              
XI=np.array([[0, 1, 2, 4, 5]])

X=np.array([[100.1,200.9,304.5,430.9,578.6]])

C1 = nan

print("J[0] =",J)

for i in J[0]:
    X = np.insert(X, i, [C1], axis=1)
print("X =", [X])

Amin=0
Amax=600.0


color = cm.get_cmap('Dark2')
norm = Normalize(vmin=Amin, vmax=Amax)
color_list = []
for i in range(len(X[0])):
    color_list.append(color((X[0,i]-Amin)/(Amax-Amin)))
    
#print(color_list)
id = 0
for j in range(0, n):
    for k in range(n-1):
        ax.hlines(200+200*(n-j-1)+5*n, 200*(k+1)+5*n, 200*(k+2)+5*n, zorder=0, linewidth=5.0,colors=color_list[id])
        id += 1

    for i in range(0, n):
        rect = mpl.patches.Rectangle((200+200*i, 200+200*j), 10*n, 10*n, linewidth=1, edgecolor='black', facecolor='black')
        ax.add_patch(rect)
        if j < n-1:
            ax.vlines(200+200*i+5*n, 200*(n-1-j)+5*n, 200*(n-j)+5*n, zorder=0, linewidth=5.0, colors=color_list[id])
            id += 1

cb = fig.colorbar(cm.ScalarMappable(cmap=color, norm=norm))
cb.set_ticks(np.arange(Amin, Amax+0.1e-05, (Amax-Amin)/6).astype(float))
cb.set_label("u0394P (N/m$^{2}$)")
ax.set_xlim(left = 0, right = 220*n)
ax.set_ylim(bottom = 0, top = 220*n)

# ax.set_yticklabels([])
# ax.set_xticklabels([])
plt.axis('off')
plt.title("Time = 0.0",fontsize=20)
plt.show()

The present output is

Present output

Asked By: AEinstein

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Source

Answer 1

The amount of colors in your chosen colormap is 8, so by default it will stretch those within your given min/max range.

You can use a BoundaryNorm to set specific ranges for each color. Since there are eight colors, it still depends a little on whether you want for example the first 6, or "maximize" over the entire colormap etc.

from matplotlib.colors import BoundaryNorm
import numpy as np

norm = BoundaryNorm(np.arange(Amin, Amax+1, 100), color.N)

The normalizer above sets the amount of colors to the length of the colormap, this causes it to sample the 6 colors (defined by the boundaries) to be stretched along the entire map, from first to last. Setting it to 6 would for example use the first 6 colors.

Using the above normalizer, the result looks like:

Answered By: Rutger Kassies

Splitting the range of colorbar in Python

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