@peterdrake @mkj @jitseniesen @apa
Well, I confirmed that the bizarro text at the top of the Y axis is not in fact part of the Y axis. If you set the style of the Y axis tick labels to plain, so it doesn't use scientific notation:
>>> plt.ticklabel_format(axis="y", style="plain")
... that one bizarro value is still shown in scientific format.
I don't recall if matplotlib supports multiple scales/sets of ticks on an axis, so I'm not sure where that value is coming from. I looked at the data generated by your code and didn't see anything weird.
I think you need either a matplotlib expert, or a user discussion group. Or at least someone who's used matplotlib more recently than the Clinton administration...