An attempt at a more general animation API

[cpsievert]

Note to self: this is really the only relevant part -- https://github.com/ropensci/plotly/pull/938/files#diff-bfe74656b9837557ae3edcc5e1daa831R22

May want to cherry-pick that code as merging might be a disaster

[cpsievert]

Also, the point of the API is to be able to transition between different trace types -- https://github.com/ropensci/plotly/blob/7f15fd68e61ab76cee2f6ce6e577aa927538cb7d/R/animate.R#L11-L16

the current API (i.e., frame) assumes the animation has a fixed trace type

[rreusser]

What does it do visually as it transitions types? That is, what does it even mean to transition a bar graph into a scatter trace? A long-running goal is to smooth out those things so they at least do something nice/reasonable.

[cpsievert]

What does it do visually as it transitions types?

I'm hoping plotly.js can work out those details ;)

More seriously, this is just an attempt to make it easier to "animate" through a collection of plots (i.e., it's ok if a redraw is required)