Violet, blue, yellow and green bars create a parallelogram.

Orange pen center, fixed pivot center of the blue bar and center of revolute joint between pink and yellow bars always create a straight line.

While the pink bar draws a cicle of radius a (length of the pink bar) the pen draws orange circle of radius R.

R/a = length ratio of the blue bar.

This avoids the need to arrange a pivot at the circle center.