View g Snap to plot (shift-F3)
This will display a highly irritating small black follow-the-mouse-cursor dot. This dot attaches itself to the data point (of any plot) that is nearest to the mouse cursor. The values you can read in the X and Y value numeric displays on the top right of the graph will be the actual values of that data point, and not the X and Y values of the mouse cursor.
If one or more data sets are selected, the dot will only snap to the selected plots.
When you click anywhere on the graph, the dot will jump away from its attached data point to the mouse cursor position. It will stay there until you move the mouse, after which it will again attach to a plot. Dragging the mouse will also cause the dot to stay attached to the mouse cursor position. This way you will still be able to determine X and Y values of points that do not belong to a plot.