How can I plot amplitude of transfer function in three dimension (for instance to check poles and zeros on graph) ? 
Suppose this is my transfer function:

![Transfer function](https://i.stack.imgur.com/IdqOf.png)

My code:

```
b = [6 -10 2];
a = [1 -3 2];

[x, y] = meshgrid(-3:0.1:3);
z = x+y*j;

res = (polyval(b, z))./(polyval(a,z));
surf(x,y, abs(res));

```

Is it correct? I'd also like to know is it possible to mark unit circle on plot?