Capacitance - Voltage characteristic diagram in a Schottky diode
OK first I've never looked at the capacitance of a Schottky diode. 
But (in theory) I would expect them to be like an abrupt PN junction and show a capacitance that decreased as the square root of the reverse voltage. (C ~ 1/V^2) 

(There is still a doped semiconductor wit ha depletion layer on one side of the junction.)  

Here's a one, 
another, 
more...

I'd have to plot up some points to see how well practice follows theory. 

Edit:  The last link from Fairchild does the C-V plot log-log. And so you can see that it sorta goes as 1/V^2.  (Change from 1 V to 100 V is about 1/10.. 200pf to 20 pf.)