Thanks for the response Mylenium,
you may have lost me already though -- let me try to be clear:
1. Yes, this about spatial parameters (position, or anchor point)
2. It's mainly about being able to ease a long move but not having the ease curve be spread out over the whole curve. For example with some curve types, like quadratic, on a 20s move the first 8 or 9 seconds are all acceleration and the last 8 or 9s are all deceleration. Even sinusoidal (the standard type) spreads out too far on long moves.
-- what I want, is a way to preserve a long section in the middle that would be flat on the speed graph. I want the easing to happen only in the first/last 2-5 seconds (a value to be specified in the expression, hopefully), regardless of the length of overall move.
I think it's the kind of thing that sounds trivial at first but it's pretty hard to do. Even with standard keyframes you can't dial it in, you have to tweak by hand.
Here's a basic move from 0 - 6 seconds, linear
Then I add two more keyframes and only ease the outer ones, hoping to preserve a linear section in the middle; obviously still not right
The finally, what the KFs should look like...actually I can't even quite get it by hand; it should be continuous speed at 1 and 5s and a flat line between them. Here's the closest I could get, only by moving the middle KFs closer together which I don't want to do and also isn't the right solution anyway