Allow adjustment by fractional pixels

[jason-s]

Sometimes when I put a marker on a point in my graph, and adjust it by one pixel at a time, it is slightly too low or too high and I would like it to be between positions.

From user manual:

After the measurement is
made, you can adjust the highlighted (green) point using the cursor keys on the keyboard
(use Shift for faster movement).

Could you also add a more fine-grained adjustment (1/2 pixel?) by making some change in a global setting, or using another modifier key (e.g. Ctrl-cursor moves by 1/2 pixel) ?