Proceeds in two steps, both of which require user input
from the mouse: 1) Read the image in and calibrate it
2) Digitize the data
Calibration points are optionally passed via arguments x1, x2, y1,
y2. These **must be named in full** if passed.
If not specified, you are prompted to enter these in the
console. Note, you don’t need to choose the end points of each axis,
only two points for which you know the x or y return.