Data Smoothing with Splines

I have R installed, but I am a rookie.  I was able to install all of the necessary packages so that the sample .pbix file provided here loads and works just fine for me.  When I try to add the spline visual to my own .pbix file it doesn't want to accept measures in the X and Y axis.  Message on the visual simply states "Need numeric X and Y variables".  They are numeric, the Data Type is "Decimal Number", but formatted as "Percentage".  Why wouldn't I be able to use measures?  The data I have works just fine with the built in Power BI "scatter chart".  What I really want is the line so that I can keep a National Average line on the visual and cross-highlight by districts to see the distance from a National Average.

Very cool visual, I hope that I can get it working.