It's just referring to the "x/y" pixel position. "x,0" is the top of the display with your baseline at position x,225.
To plot the Temp against this baseline we simply deduct the temperature from 225. If the temperature was 0 then the graph would show a line being drawn at x,225 (225-0).
If the temperature was 25 then the graph would show a line being drawn at x,200 (225-25)
Nothing magical I'm afraid

Regards