When users select one text column to create a Pie Chart (including 3D Pie) or Doughnut plot, we show the following error message:

To count the unique value with pie/doughnut chart, 
    1. Add one Y column with F(x)=1.
    2. Select XY columns and create plot.

This is meant to help the user understand that some counting (using a numeric column) must be done first. Once that is completed, the plot can be made.