This makes a nonlinear cap and graphs c vs v. It's a shot at simulating a Z5U ceramic cap. It can be tweaked to match some actual cap curve.
The '2' factor can go up to make the dropoff steeper. And you can always put a fixed cap in parallel to set the floor.
Version 4 SHEET 1 1096 680 WIRE 48 80 0 80 WIRE 176 80 48 80 WIRE 320 80 240 80 WIRE 480 80 320 80 WIRE 576 80 528 80 WIRE 624 80 576 80 WIRE 0 144 0 80 WIRE 320 144 320 80 WIRE 528 144 528 80 WIRE 480 160 480 80 WIRE 0 272 0 224 WIRE 320 272 320 224 WIRE 480 272 480 208 WIRE 528 272 528 224 FLAG 0 272 0 FLAG 320 272 0 FLAG 480 272 0 FLAG 528 272 0 FLAG 576 80 CAP FLAG 48 80 RAMP SYMBOL res 304 128 R0 WINDOW 0 49 47 Left 2 WINDOW 3 50 78 Left 2 SYMATTR InstName R1
SYMBOL voltage 0 128 R0 WINDOW 0 68 65 Left 2 WINDOW 3 17 112 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PULSE(0 100 0 100) SYMBOL e 528 128 R0 WINDOW 0 64 53 Left 2 WINDOW 3 58 85 Left 2 SYMATTR InstName E1 SYMATTR Value 1e6 SYMBOL cap 176 96 R270 WINDOW 0 -43 32 VTop 2 WINDOW 3 77 35 VBottom 2 SYMATTR InstName C1 SYMATTR Value q = x / (1+2*x) TEXT 16 112 Left 2 ;1 V/S TEXT 560 112 Left 2 ;1 V / F TEXT 368 8 Left 2 !.tran 0 1 0 TEXT 8 -96 Left 2 ;Nonlinear Capacitor C:V Curve TEXT 48 -56 Left 2 ;J Larkin August 11, 2014 TEXT 480 -96 Left 2 ;Cap is 1F at low voltage TEXT 536 -56 Left 2 ;0.1F at 1 volt