inspection of displayed plot data

A very common feature for the inspection of displayed data is something that might be called cursor/tracker. It offers information about a selected point or coordinate ...

F.e dragging a crosshair might give some information about a position ( coordinates, coordinates of the closest curve points etc ). A vertical line might offer information about the y coordinates of the displayed curves for a specific x coordinate ( or v.v. )

The Qwt bode example show such a crosshair ( press the left button ), while the Qwt curvetracker demo shows an example of the second scenario.