Calculating similarity between two signal plots
Why not just use something like the relative "error" between the two?

For example, if your frequency magnitude responses are $G_1$ and $G_2$, then calculate:

$$
ERR = \sum \left | G_1(n) - G_2(n) \right|^2
$$

and then normalize with respect to the reference, $G_1$:

$$
NORMALIZED = \sum \left | G_1(n) - G_2(n) \right|^2 /  \sum \left | G_1(n) \right|^2
$$

Correlation is also a way to go, but it might show some cases where the same shape happens, but at very different frequencies... which might not be what you want.