Need a suggestion for an application to do line drawing with cartesian coordinates.

Specifically: I would like to draw lines with the mouse on a cartesian grid and be able to read coordinates of the points to the 2nd or 3rd decimal place.

I need to produce images for TeX and enter these points using the Tikz package. The points are not produced from mathematic operations, they're products of imagination.

I could put a grind in Gimp and add another layer to make the drawings, but I would be guessing on the decimal points.

Any suggestions?