You can mark any number of points on a curve in order to identify certain positions. First place the cursor at the desired position. Then open the context menu with the right mouse button and select the Set Mark command. Each mark includes the value measured at that position. If there is more than one mark, then sequential numbers are shown.
In order to remove a mark, position the cursor near the arrow point of the mark, and from the context menu select the Remove Mark command, or select CurveĀ > Remove All Marks from the main menu.
The shortcut CTRL+M can also be used to set a mark, or to remove one if the cursor is near a mark.