I need to create an XY plot of the data from 2 tags but am having difficulty doing so. I have looked at the PlotXY function but am unsure of where to use it and how to get the arrays of data needed for the function.

I have never had a need to write code to build trends in VTScada.

Almost everything desirable can be accomplished by adding pens and saving a tag group for more than 1 tag on a custom trend or using the ad-hoc trend which popups up by clicking a tag drawn with the Numerical Value tag.

If you are looking to draw a trend on screen, there's also the Plot Data widget
https://www.vtscada.com/help/Content/D_Widgets/Dev_PlotDataWidget.htm?Highlight=plot

So let's call one tag's data X and the other tag's data Y. Would the solutions you mentioned be able to plot X against Y? Or only X and Y against time?

Normally, for SCADA, it should all be plotted/trended against time. But if you need to plot value x against y, like x=ay+b formula. You can just create a calculated tag with that formula, and trend that tag against time.