Support dynamic fillValue values

[micheleb]

Fix #103

[Ofekino]

@micheleb Hi mate, this works great, I'm just wondering:
When I update the value to be higher than it was, the bar is animating correctly (interpolating to the new value).
However, when the new value is lower than it was, it instantly snaps to the new value without animating.
Do you know how to fix that?
Thank you!