ilinear(t) := (1-t)*a + t*b;

P:  [ [2, 3], [3,2], [4,3], [5,2]]$

x12(t) := ev(ilinear(t), a=P[1][1], b=P[2][1])$
y12(t) := ev(ilinear(t), a=P[1][2], b=P[2][2])$
x23(t) := ev(ilinear(t), a=P[2][1], b=P[3][1])$
y23(t) := ev(ilinear(t), a=P[2][2], b=P[3][2])$
x34(t) := ev(ilinear(t), a=P[3][1], b=P[4][1])$
y34(t) := ev(ilinear(t), a=P[3][2], b=P[4][2])$

x123(t) := ev(ilinear(t), a=x12(t), b=x23(t))$
y123(t) := ev(ilinear(t), a=y12(t), b=y23(t))$
x234(t) := ev(ilinear(t), a=x23(t), b=x34(t))$
y234(t) := ev(ilinear(t), a=y23(t), b=y34(t))$

x1234(t) := ev(ilinear(t), a=x123(t), b=x234(t))$
y1234(t) := ev(ilinear(t), a=y123(t), b=y234(t))$

curvas: [
    [discrete, P],
    [parametric, x123, y123, [t, 0, 1] ], 
    [parametric, x234, y234, [t, 0, 1] ], 
    [parametric, x1234, y1234, [t, 0, 1] ]
]$
plot2d(curvas, [x, 1.8, 5.2], [y, 1.8, 3.2], [legend, "Polígono", "P_{123}", "P_{234}", "P_{1234}"]);