Now supports updating axis:

fig, ax = plt.subplots()
ax.plot(['a', 'c', 'e'], label="plot 1")
ax.plot(['a', 'b', 'd'], label="plot 2")
ax.plot(['b', 'e', 'd'], label="plot3")
ax.legend()

Which as a sidenote, it didn't work before because apparently FixedLocator converts to an array as soon as it's passed in (and also apparently it doesn't need to...)

UnitData was also changed into an object to facilitate the updating and as a precursor to adding more functionality. And the tests should now be py.test compliant @Kojoley