One more thing....
After re-reading your message, I noticed where you said he was "shaking violently". I have seen Chameleons do this too. I'm not scientifically sure, but I have a theory that this behavior is not actually an indication of displeasure with the spraying.
What color is he when he does it? If he stays light green, and doesn't get deep olive or black, he probably isn't mad. Does he open his mouth?
My theory is that this is him "stretching", kind of like you do sometimes when you wake up. I think it's kind of like he's "resetting" his muscles,
and the spraying stimulates him to shudder. Have you ever stretched so hard that you shuddered a bit? Anyway, that's just a theory. If his color is normal, then he's probably not mad. This behavior may actually be an expression of pleasure, even though it looks like he's angry. Besides, would a Chameleon get "angry" if he got rained on in the wild? Probably not. And Jackson's are native to mountains, where the rain is probably pretty cold.
Who knows, it's just a theory. Maybe it really does piss them off! I've seen Chameleons do this a lot. So I know it's nothing to worry about.