Image Ruler

Tool to measure horizontal and vertical lines drawn on an image.
Lengths are converted to a custom unit (ex: inches, cm) by calibrating to a line of a known length.

Usage:

1. import an image
2. choose an axis
3. calibrate:
   1. draw a line (click, drag, release)
   2. enter the length of this line in a custom unit
   3. click: "Calibrate"
4. measure:
   1. draw another line along the same axis
   2. read its length, as measured in the custom unit