WIP: Multiple y-axis formatting

[hannahker]

A simpler starting implementation of #6319, where we assume a constant offset (xshift) applied to anchored y axes that may be set to either the left or right side of a plot.

To-do's:

• [ ] Offset axes based on width of previously drawn axes on the same side
• [ ] Remove need for automargin=True to be set for the first y-axes. This is causing multiple redraws of the axes, but without it the GetLinePosition() function gets called before the axis is drawn and so there isn't any xshift to be applied.
• [ ] Address the Error: <text> attribute x: Expected length, "NaN" errors (although I can't find any impact of this on the output plot.
• [ ] Fix all the failing tests :(