A page-wide reticle centered at the pointer location to easily read plot axes. Especially useful to digitize graph data.

Extra feature: Click, and the reticle stays centered there, allowing one to zoom in/out to read axis for instance. Click again to free the reticle.

Extra feature: Angle of the reticle should be customizable in the case of tilted scanned documents. Width of the reticle line as well, why not :)