OptionaldurationSeconds this leg lasts. Required unless spring supplies one.
OptionaleaseEasing for this leg. Mutually exclusive with spring.
OptionalholdSeconds to rest at to before the next step begins.
OptionalspringSpring physics for this leg instead of an easing. Supplies its own duration.
Value at the end of this step. The start is wherever the previous step finished.
One leg of a sequence: where to move to, over how long, and how.