python - Plot two histograms on single chart with matplotlib

Here you have a working example:

import random import numpy from matplotlib import pyplot  x = [random.gauss(3,1) for _ in range(400)] y = [random.gauss(4,2) for _ in range(400)]  bins = numpy.linspace(-10, 10, 100)  pyplot.hist(x, bins, alpha=0.5, label='x') pyplot.hist(y, bins, alpha=0.5, label='y') pyplot.legend(loc='upper right') pyplot.show() 

The accepted answers gives the code for a histogram with overlapping bars, but in case you want each bar to be side-by-side (as I did), try the variation below:

import numpy as np import matplotlib.pyplot as plt plt.style.use('seaborn-deep')  x = np.random.normal(1, 2, 5000) y = np.random.normal(-1, 3, 2000) bins = np.linspace(-10, 10, 30)  plt.hist([x, y], bins, label=['x', 'y']) plt.legend(loc='upper right') plt.show() 

Reference: http://matplotlib.org/examples/statistics/histogram_demo_multihist.html

EDIT [2018/03/16]: Updated to allow plotting of arrays of different sizes, as suggested by @stochastic_zeitgeist

In the case you have different sample sizes, it may be difficult to compare the distributions with a single y-axis. For example:

import numpy as np import matplotlib.pyplot as plt  #makes the data y1 = np.random.normal(-2, 2, 1000) y2 = np.random.normal(2, 2, 5000) colors = ['b','g']  #plots the histogram fig, ax1 = plt.subplots() ax1.hist([y1,y2],color=colors) ax1.set_xlim(-10,10) ax1.set_ylabel("Count") plt.tight_layout() plt.show() 

In this case, you can plot your two data sets on different axes. To do so, you can get your histogram data using matplotlib, clear the axis, and then re-plot it on two separate axes (shifting the bin edges so that they don't overlap):

#sets up the axis and gets histogram data fig, ax1 = plt.subplots() ax2 = ax1.twinx() ax1.hist([y1, y2], color=colors) n, bins, patches = ax1.hist([y1,y2]) ax1.cla() #clear the axis  #plots the histogram data width = (bins[1] - bins[0]) * 0.4 bins_shifted = bins + width ax1.bar(bins[:-1], n[0], width, align='edge', color=colors[0]) ax2.bar(bins_shifted[:-1], n[1], width, align='edge', color=colors[1])  #finishes the plot ax1.set_ylabel("Count", color=colors[0]) ax2.set_ylabel("Count", color=colors[1]) ax1.tick_params('y', colors=colors[0]) ax2.tick_params('y', colors=colors[1]) plt.tight_layout() plt.show() 

As a completion to Gustavo Bezerra's answer:

If you want each histogram to be normalized (normed for mpl<=2.1 and density for mpl>=3.1) you cannot just use normed/density=True, you need to set the weights for each value instead:

import numpy as np import matplotlib.pyplot as plt  x = np.random.normal(1, 2, 5000) y = np.random.normal(-1, 3, 2000) x_w = np.empty(x.shape) x_w.fill(1/x.shape[0]) y_w = np.empty(y.shape) y_w.fill(1/y.shape[0]) bins = np.linspace(-10, 10, 30)  plt.hist([x, y], bins, weights=[x_w, y_w], label=['x', 'y']) plt.legend(loc='upper right') plt.show() 

As a comparison, the exact same x and y vectors with default weights and density=True:

You should use bins from the values returned by hist:

import numpy as np import matplotlib.pyplot as plt  foo = np.random.normal(loc=1, size=100) # a normal distribution bar = np.random.normal(loc=-1, size=10000) # a normal distribution  _, bins, _ = plt.hist(foo, bins=50, range=[-6, 6], normed=True) _ = plt.hist(bar, bins=bins, alpha=0.5, normed=True)