As a dynamic extension to what Jim recommends, my favorite way to do this is as follows:
1) add a new column to the table, name it "Selected", make it nominal, and add to it the formula
Selected ( Row State ( ) )
This column will have 1 for any selected row, and 0 for any unselected row, and will update automatically as you make various selections.
2) In graph builder, drag this new "Selected" column to the x-axis.
If you have no rows selected in your table, you'll get something like this:
3) As soon as you select anything (even within this very graph), you'll get this:
Exactly HOW you make the selection, is completely up to you. You can make the selection whatever you wish, and you'll always see what you've selected, vs everything else.