Trying to make a graphical interface to biquad~, not working

So here's what I want to do:

I want to have a spectrograph of an audio signal, and be able to graphically isolate a frequency range, and pass that to a biquad~. From there, I derive a floating point value between 0 and 1, and use it to drive another program via OpenSoundControl.

I thought I had nailed it by making a filtergraph~ transparent and layering it on top of a spectroscope~ object. However, it doesn't seem like the filtergraph~ is actually doing anything. It seems like the patch is only responding to overall amplitude. (Everything other than the biquad~ works as expected).

What am I doing wrong?

Here's my patch: