Green and blue gears have same tooth number.
COD: straight line
AC and BD are parallel.
Triangles OAC and OBD are similar.
OC/OD = OA/OB = AC/BD = k = constant
Figures traced by pen D and pen C are similar but upside down each other. Scale factor is k.
Adjust OA and AC to get various values of k.
Bar linkage in a conventional pantograph is replaced by rack pinion drive.