Iris documentation - example test mpl UserWarning

[bjlittle]

The iris documentation example test docs/iris/example_tests/test_COP_1d_plot.py, which calls docs/iris/example_code/Meteorology/COP_1d_plot.py, causes the following matplotlib 2.2.2 warning to be raised:

<snip>/lib/python2.7/site-packages/matplotlib/ticker.py:1853: UserWarning: Steps argument should be a sequence of numbers
increasing from 1 to 10, inclusive. Behavior with
values outside this range is undefined, and will
raise a ValueError in future versions of mpl.

This is being generated by matplotlib itself, through it's own default choice of tick locations using matplotlib.ticker.MaxNLocator and the example cube data. The entry point to matplotlib from iris is from iris.plot._draw_1d_from_points - btw no string axes are involved in the plot.

We should investigate this and circumvent within iris through our own choice of tick locator, or determine if this is an issue within matplotlib that requires fixing for the community.

[bjlittle]

As it happens, see nc-time-axis PR #34...

[pelson]

nc_time_axis>=1.1 should fix.