Drawing average line in histogram (matplotlib)

You can use plot or vlines to draw a vertical line, but to draw a vertical line from the bottom to the top of the y axis, axvline is the probably the simplest function to use. Here's an example:

In [80]: import numpy as np

In [81]: import matplotlib.pyplot as plt

In [82]: np.random.seed(6789)

In [83]: x = np.random.gamma(4, 0.5, 1000)

In [84]: result = plt.hist(x, bins=20, color='c', edgecolor='k', alpha=0.65)

In [85]: plt.axvline(x.mean(), color='k', linestyle='dashed', linewidth=1)
Out[85]: <matplotlib.lines.Line2D at 0x119758828>

Result:

This is old topic and minor addition, but one thing I have often liked is to also plot mean value beside the line:

import numpy as np
import matplotlib.pyplot as plt
np.random.seed(6789)
x = np.random.gamma(4, 0.5, 1000)
result = plt.hist(x, bins=20, color='c', edgecolor='k', alpha=0.65)
plt.axvline(x.mean(), color='k', linestyle='dashed', linewidth=1)

min_ylim, max_ylim = plt.ylim()
plt.text(x.mean()*1.1, max_ylim*0.9, 'Mean: {:.2f}'.format(x.mean()))

Which produces following result: