In the windows where you create the expressions, select the second one and click on GROUP.
After this, you will have a new icon on the chart. It looks like a loop or the refresh icon of a web browser.
When you left-click it, it goes to the next expression. With the right click you can select the expression that you want.
You can move this icon by holding Ctrl+Shift.
I like to put it on the top, near the title. I think it's easier for the users there.
Hope it helps.