When there are several curves in the plot, it is sometimes useful to display only one of the curves temporarily without deleting the others. This can be done with the button on the toolbar, or from Plot->Solo Curve.
When Solo is on, only the curve belonging to the active measurement is visible. The other curves will be invisible in the plot, but they are not deleted from the legend list. If Solo is turned off, all curves that were displayed when Solo was turned on will be displayed again.