This is a minimal example and produces the desired result:
import matplotlib.pyplot as plt
import numpy as np
data_to_plot = np.random.rand(100,5)
fig = plt.figure(1, figsize=(9, 6))
ax = fig.add_subplot(111)
bp = ax.boxplot(data_to_plot, showmeans=True)
plt.show()
EDIT:
If you want to achieve the same with matplotlib version 1.3.1 you'll have to plot the means manually. This is an example of how to do it:
import matplotlib.pyplot as plt
import numpy as np
data_to_plot = np.random.rand(100,5)
positions = np.arange(5) + 1
fig, ax = plt.subplots(1,2, figsize=(9,4))
# matplotlib > 1.4
bp = ax[0].boxplot(data_to_plot, positions=positions, showmeans=True)
ax[0].set_title("Using showmeans")
#matpltolib < 1.4
bp = ax[1].boxplot(data_to_plot, positions=positions)
means = [np.mean(data) for data in data_to_plot.T]
ax[1].plot(positions, means, 'rs')
ax[1].set_title("Plotting means manually")
plt.show()
Result:
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