I wrote this expression initially to make some images shrink for a slideshow
but I've used it a lot to animate between point a and b when the frame is added to the timeline.
Hope someone else finds it useful too!
//Expand an object from 0 to 200% over 19.2 seconds - apply to %
rate = 19.2;
expand = linear(time, inPoint, inPoint+rate, 0, 200);