Bug report

Sketch transformation has identical pseudo-random output for identical inputs

The sketch transformation (such as used in xkcd) produces exactly the same randomized result if the same path is plotted repeatedly. This is visible especially in the legend, where the rectangle patch is repeated for each item. I exaggerated randomness and scale to make the effect better visible.

As far as I can see, the culprit is in the built-in RNG which gets re-seeded to 0 for each Sketch instantionation:

import matplotlib.pyplot as plt
with plt.xkcd(randomness=40,scale=1):
    import matplotlib
    from matplotlib import gridspec
    pat,txt=plt.pie([10,20,30,40],wedgeprops={'edgecolor':'black'})
    plt.legend(pat,['first','second','third','fourth'],loc='best')

Actual outcome

zoom on the legend:

Expected outcome

Shapes are distored randomly, e.g. each rectangle in the legend is different.

Matplotlib version

• Operating system: Ubuntu 18.04
• Matplotlib version: 2.2.2
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg
• Python version: 3.6.7
• Jupyter version (if applicable): [none]
• Other libraries:

installation: default packaged matplotlib installation (python3-matplotlib).