I certainly do not know, but I it seems that this behavior is very strange.plt.xticks does not accept labels which they return as arguments. As a simple example of this odd behavior:

x = np.arange(0,10)plt.plot(x,x)locs, labs = plt.xticks()plt.xticks(locs[1:], labs[1:])plt.show()

matplotlib 1.5.3
python 3.5.2
win 10