Python: Creating a 2D histogram from a numpy matrix

Question

I\'m new to python.

I have a numpy matrix, of dimensions 42x42, with values in the range 0-996. I want to create a 2D histogram using this data. I\'ve been looking a

Answer 1

@unutbu's answer contains a mistake: xidx and yidx are calculated the wrong way (at least on my data sample). The correct way should be:

xidx = np.clip(np.digitize(x, xedges) - 1, 0, hist.shape[0] - 1)
yidx = np.clip(np.digitize(y, yedges) - 1, 0, hist.shape[1] - 1)

As the return dimension of np.digitize that we are interested in is between 1 and len(xedges) - 1, but the c = hist[xidx, yidx] needs indices between 0 and hist.shape - 1.

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Below is the comparison of results. As you can see you get similar but not the same result.

import numpy as np
import matplotlib.pyplot as plt

fig = plt.figure()
ax1 = fig.add_subplot(211)
ax2 = fig.add_subplot(212)

n = 10000
x = np.random.standard_normal(n)
y = 2.0 + 3.0 * x + 4.0 * np.random.standard_normal(n)
xedges, yedges = np.linspace(-4, 4, 42), np.linspace(-25, 25, 42)
hist, xedges, yedges = np.histogram2d(x, y, (xedges, yedges))

xidx = np.clip(np.digitize(x, xedges), 0, hist.shape[0] - 1)
yidx = np.clip(np.digitize(y, yedges), 0, hist.shape[1] - 1)
c = hist[xidx, yidx]
old = ax1.scatter(x, y, c=c, cmap='jet')

xidx = np.clip(np.digitize(x, xedges) - 1, 0, hist.shape[0] - 1)
yidx = np.clip(np.digitize(y, yedges) - 1, 0, hist.shape[1] - 1)

c = hist[xidx, yidx]
new = ax2.scatter(x, y, c=c, cmap='jet')


plt.show()