Plot histogram with colors taken from colormap

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1

I was able to get the “jet” color map working with my scatter plot, but not with my histogram. Could someone with fresh eyes tell me what I’m doing wrong? Thanks in advance.

import numpy as np
from matplotlib import pyplot as plt
from astropy.visualization import hist
from astroML.datasets import fetch_nasa_atlas

data = fetch_nasa_atlas()

#------------------------------------------------------------
# plot the RA/DEC in an area-preserving projection

RA = data['RA']
DEC = data['DEC']

# convert coordinates to degrees
RA -= 180
RA *= np.pi / 180
DEC *= np.pi / 180

ax = plt.axes(projection='mollweide')
plt.scatter(RA, DEC, s=1, c=data['Z'], cmap=plt.cm.jet,
            edgecolors='none', linewidths=0)
plt.grid(True)

plt.title('NASA Atlas Galaxy Locations')
cb = plt.colorbar(cax=plt.axes([0.05, 0.1, 0.9, 0.05]),
                  orientation='horizontal',
                  ticks=np.linspace(0, 0.05, 6))
cb.set_label('redshift')


#------------------------------------------------------------
# plot the r vs u-r color-magnitude diagram

absmag = data['ABSMAG']

u = absmag[:, 2]
r = absmag[:, 4]

plt.figure(figsize=(14,10))
#plt.figure()
ax = plt.axes(facecolor='#000042')
plt.scatter(u - r, r, s=1, lw=0, c=data['Z'], cmap=plt.cm.jet)
plt.colorbar(ticks=np.linspace(0, 0.05, 6)).set_label('redshift')

plt.xlim(0, 3.5)
plt.ylim(-10, -24)

plt.xlabel('u-r')
plt.ylabel('r')

#------------------------------------------------------------
# plot a histogram of the redshift

# This is  the colormap I'd like to use.
cm = plt.cm.get_cmap('jet')

plt.figure(figsize=(12,9))
n, bins, patches = hist(data['Z'], bins='knuth', histtype='stepfilled', ec='k')
plt.xlabel('z')
plt.ylabel('N(z)')

#print("n: ", n, "\n\nbins: ", bins, "\n\npatches: ", patches)

# https://www.tutorialspoint.com/plot-a-histogram-with-colors-taken-from-colormap-in-matplotlib
col = (n-n.min())/(n.max()-n.min())
#print("\ncols: ", col)

for c, p in zip(col, patches):
   plt.setp(p, 'facecolor', cm(c))

plt.show()

redshift_scatter.PNG
redshift_scatter.PNG886×689 72.5 KB

redshift_histogram
redshift_histogram868×620 18.2 KB

2

Isn’t step filled just returning one patch? You probably want to switch to bar if you want to color each bar separately.

3

To expand on what @jklymak said, if you use histtype='stepfilled', then we return a single patch that is the full histogram. If you want it to all be the same color and to have small (on the screen) bins, this can lead to massive performance gains as we only have to draw one polygon (which has the the shape of the outline of your histogram) even though you read it as having many bars. In this case patches is length 1 and zip happily terminates after just one pass through the loop (check the length of patches and put a print in that loop to check).

If you want to color each bar idepnedently, then we need to draw them independently. Changing to something like

import matplotlib.pyplot as plt
import numpy as np
fig, ax = plt.subplots()
cm = plt.get_cmap('turbo')
edges, bin, patches = ax.hist(np.random.randn(50))
for j, p in enumerate(patches):
    print(f'setting color on bar {j} ')
    p.set_facecolor(cm(j / len(patches)))
plt.show()

gives:

so

Unfortunately there is some type instability in the return values of ax.hist (see matplotlib.axes.Axes.hist — Matplotlib 3.5.1 documentation ) but that is so that we can support a wide range of inputs.