Add "Set size" or similar xlabel on set totals plot

[jnothman]

[RafaelWO]

FYI: If one wants to add this manually, the following works:

from matplotlib import pyplot as plt
from upsetplot import generate_counts, plot

example = generate_counts()

axes = plot(example)
axes['totals'].set_xlabel("Set size")
plt.show()

[jnothman]

Now an example in the example gallery