a = 2 (slider from 0 to 6, step 0.5)
D = Intersect(xAxis, yAxis)
A: (a, 0.0)
B: (0.0, 6.0 - x(A))
C: (x(A), y(B))
poly1, a_1, c, b, d = Polygon(A, C, B, D)