Replace deprecated IndexFormatter

[stephenworsley]

📰 Custom Issue

Matplotlib has deprecated the IndexFormatter class matplotlib/matplotlib#16170 is used in only one place in the Iris code:

iris/lib/iris/plot.py

Lines 251 to 256 in a2f49c3

	for axis, ticks in string_axes.items():
	formatter = mpl_ticker.IndexFormatter(ticks)
	locator = mpl_ticker.MaxNLocator(integer=True)
	this_axis = getattr(ax, axis)
	this_axis.set_major_formatter(formatter)
	this_axis.set_major_locator(locator)

A suggestion is given here for how to replace IndexFormatter with a given example matplotlib/matplotlib#16631 (comment).

[stephenworsley]

It seems as though Iris is setting its own underlying values in the case of string coordinates on this line:

iris/lib/iris/plot.py

Line 441 in a2f49c3

values = np.arange(data.shape[data_dim] + 1) - 0.5

These values seem to be anticipating the behaviour of IndexFormatter and this line would have to change if we adopted the formatter described in the above link.
After trying out the formatter described in the above link, the ticks remained in a sensible place and were all labeled correctly (in fact arguably more correctly, since there wasn't a duplicate label at the -1 position).

[rcomer]

Another comment further down that issue suggests using FixedLocator and FixedFormatter. I think that may be simpler than the FuncFormatter option.

[bjlittle]

@stephenworsley This is the same as #3822, right? 🤔

[stephenworsley]

@bjlittle yes you're right, I hadn't spotted that one.