Plotting: restore xyincrease kwarg default to True

[fmaussion]

• Closes Plot2D no longer sorts coordinates before plotting #2422
• Tests added
• Tests passed
• Fully documented, including whats-new.rst

#2294 introduced the new xincrease and yincrease kwargs and a behavior documented as:

    xincrease : None, True, or False, optional
        Should the values on the x axes be increasing from left to right?
        if None, use the default for the matplotlib function.

The default was None, I suggest to set it to True: it was the default before #2294 and I think it is what most users expect.

[dcherian]

LGTM. Thanks!

[shoyer]

thanks!