As you can see, at the top the retimer tool has an easy to access visibility toggle for loc, rot and scale. The snap to frames check box keeps keys on frames. Under those you can then choose from where you wish to scale from, use the reset button to return. Bake and zero commits the keys and puts the slider back to zero. The Retimer works in the Graph Editor, Dope Sheet and Timeline too.

Holding shift smooths the operation of the slider.