PRNO contains non-numeric, discrete values. But a scatter chart needs an aggregation of numerical values as graphical dimension (first expression in chart). A scatter chart uses the first and second expression as graphical axes.
If you want graphical axis with discrete values, you should choose a grid chart instead. I suggest that you try a grid chart with just two dimensions and one expression and take it from there. Then the two dimensions will serve as graphical axes.
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