All coordinates related to canvases are stored as floating-point
Coordinates and distances are specified in screen units,
which are floating-point numbers optionally followed
by one of several letters.
If no letter is supplied then the distance is in pixels.
If the letter is m then the distance is in millimeters on
the screen; if it is c then the distance is in centimeters;
i means inches, and p means printers points (1/72 inch).
Larger y-coordinates refer to points lower on the screen; larger
x-coordinates refer to points farther to the right.