Compare

In [1]: plt.plot([0, .2]); plt.ylim()
Out[1]: (0.0, 0.20000000000000001)

In [2]: plt.plot([-.1, .2]); plt.ylim()
Out[2]: (-0.10000000000000001, 0.25)

Note how the upper ylim changes from 0.2 to 0.25. This behavior is even tested (indirectly) by test_axes:test_vline_limit. I intend to change this behavior in my work on #5738/#5755 so that the upper limit is actually 0.2 in both cases (basically relying on matplotlib.ticker.Base, which has been written exactly to handle this).

Thoughts?