Blue links have the same length.
Green links have the same length.
Blue and green links create a rhombus.
Move violet tracer along a given curve (a circle in this case), pink pen traces pink curve (an ellipse in this case). It is the given curve scaled only in Y direction.
Y-scale factor: ky = B / (B – 2G)
B: length of the blue link
G: length of the green link
X-scale factor: kx = 1