A parts-of-whole graph can only plot one data set. To plot a different data set, double click on the graph to bring up the Format Graph dialog, go to the second tab, and choose the table and data set to plot.

Note that the values that comprise a parts-of-whole graph are always entered into one data set column. If your values are in a row, you'll need to select them, copy to the clipboard, go to an empty row and paste.