Animate trace entrance/exit

[rreusser]

This stackoverflow question makes it clear that people may expect Plotly.animate to animate the entrance of traces. It wasn't the focus, so right now it only animates existing traces. I think it's a reasonable thing people will definitely expect though, so opening this for discussion as a feature request.

See also: possible connection to #920 regarding entrance/exit of traces.

[gvwilson]

Hi - this issue has been sitting for a while, so as part of our effort to tidy up our public repositories I'm going to close it. If it's still a concern, we'd be grateful if you could open a new issue (with a short reproducible example if appropriate) so that we can add it to our stack. Cheers - @gvwilson