It seems categorical plots are limited to show 10 categories correctly when single letters are used as categories. The eleventh category is shown at the place of the second one.

import matplotlib.pyplot as plt

x = list("ABCDEFGHIJK")
y = range(len(x))
plt.plot(x,y, marker="o")

plt.show()

In case at least one of the categories has more letters, x.append("WORD"), the plot is shown correctly:

(This issue was brouht up in this Stackoverflow question)
[Produced with python 2.7, matplotlib 2.1, numpy 1.13.0rc2]