"...my design has an AGC in a FM amplifier..."
I assume that you mean AGC = Automatic Gain Control (variable gain for output amplitude control under various input conditions).
Most wideband RF amplifiers (preampliers, for receive) do not use AGC for a very simple reason...
If someone is parked next to 104.3 MHz and that nearby signal is huge, then the AGC circuit (not having any idea what station he's actually listening to), will turn down the gain, and the already-weak 88.1 MHz station that he is listening to will fade away for ** no apparent reason **.
Perhaps having just a little bit of AGC would do more good than harm, but much more than a few dB of variable gain is perhaps going to cause more problems than it solves.
The better approach is to make the preamplifier so that it has an incredibly high dynamic range and stays linear even when parked next to 104.3 MHz.
Now if the automotive radio could somehow communicate the frequency information (station being listened to) back to the antenna, and the antenna contained narrow active filters, then AGC would be perfect and the resultant system would be the best car radio antenna in the world.
All the theory and industry standard plots typically assume constant gain (not AGC). To describe your preamplifer_with_an_AGC, you're going to have to run multiple plots (perhaps printed with different colors) to describe what happens at different amplitudes.
And you should plot the results for "two tone" (widely spaced, in band) to see the potential disadvantages (side effects) of preamplifier AGC.