Affinograph.

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