This allows up to 3 Y axes to be used, one on the left side, and up to
2 on the right. To use multiple axes, one of the `ImPlotFlags_Y2Axis`
or `ImPlotFlags_Y3Axis` must be passed to `BeginPlot`. Then, before
each Plot call, SetPlotYAxis may be used to set that plot on an
alternate axis.
An extra line and ticks are rendered off to the right if all 3 axes
are configured (although those ticks are always only 5 long,
regardless of major status). Each of the axes, when present, has a
(possibly invisible if ticks and labels are disabled) hit target that
can be used to scroll and scale that axis independently.
Some other changes were required to make this happen:
* Queries are now only in pixel coordinates
* ImPlotRange has been renamed to ImPlotBounds (and correspondingly
GetPlotRange is now GetPlotBounds)
* All APIs which allow querying values in plot coordinates now have
an optional "y_axis" argument, which can be -1 to use the currently
selected axis, or non-negative to pick a specific axes.
* BeginPlot used to lock all flags on the first call. Now it only
locks the axis flags, and not the primary plot flags.
* The mouseover text now renders the 2nd and 3rd axis in parenthesis.