PR Summary

Closes #11337

The following test would fail because the view limit was set from 0-1:

    x = np.array([datetime.datetime(2017, 1, n) for n in range(1, 6)])
    y = np.array([1,2,3,4,5])
    m = np.ma.masked_greater(y, 0)

    fig = plt.figure()
    ax = fig.add_subplot(111)
    ax.plot(x, m)
    # these are the default viewlim
    assert ax.get_xlim() == (730120.0, 733773.0)

The issue was in autoscale_view, which never checked the axes.viewLim, even if the data lim came back undefined. This PR checks the viewlim and uses that if the datalim is not finite.

Note this issue is more generic than dates, but is most easily seen there because we can't have dates with ordinal values <1, and the default behaviour made it so such dates were the datalim.

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way