MATLAB Plot vertical line

Question

I have a system of equations, about 3 equations, that I need to plot that are dependent on 2 variables, R and z. But one of the equations is R={constant value}. Here's the code:

R = 0:.1:500;
z = 5./R;
z2 = 40./R;
z3 = 20
plot(R, z, R, z2, R, y3)

...I don't really know how to get z3 to show up as a vertical line. Any ideas?

Answer 1

z3 not showing up is because it has different size with R. In plot command two vectors must share the same length.

A simple solution is:

R = 0.1:.1:500;
z = 5./R;
z2 = 40./R;
z3 = 20;
plot(R, z, R, z2)
hold on;
plot([z3 z3], [min([z(:);z2(:)]) max([z(:);z2(:)])]);

Notice here 0 is excluded for otherwise max will give an Inf.