axes.format_{x,y} data should have a setter #17286

In the Date tick labels example, the code for setting the cursor coords message overwrites the default formatters:

ax.format_xdata = mdates.DateFormatter('%Y-%m-%d')
ax.format_ydata = lambda x: '$%1.2f' % x  # format the price.

This creates confusion because runs counter to how basically everything else in matplotlib is set via setter but especially the formatters:

ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y'))

and the docs aren't especially clear either on this being a property:

Axes.format_xdata(self, x)

    Return x formatted as an x-value.

    This function will use the fmt_xdata attribute if it is not None, else will fall back on the xaxis major formatter.

To be consistent with the API, I'm proposing properties/functions of the form:

• ax.{x,y}data_formatter(mdates.DateFormatter('%Y-%m-%d'))
• ax.data_formatter(which=(x,y, both), mdates.DateFormatter('%Y-%m-%d'))