Animation frames must all have the same number of traces

[nzjrs]

Relevant pen: https://codepen.io/anon/pen/KXaGda

We'd like to add an remove traces in each animation frame, however AFAICT plotly requires the number of traces to be constant across all frames. Changing the number of traces in an animation frame gives the following error

Uncaught (in promise) TypeError: Cannot read property '_module' of undefined
    at Object.plots.transition (plotly-latest.js:151918)
    at nextFrame (plotly-latest.js:132610)
    at doFrame (plotly-latest.js:132654)
    at beginAnimationLoop (plotly-latest.js:132658)
    at queueFrames (plotly-latest.js:132576)
    at plotly-latest.js:132771
    at new Promise (<anonymous>)
    at Plotly.animate (plotly-latest.js:132505)
    at Object.exports.executeAPICommand (plotly-latest.js:142494)
    at plotly-latest.js:121413

Is this in fact the expected behaviour? I didn't see this limitation documented anywhere. Do you have any ideas how to work around this?

[etpinard]

Related to #932 I believe.

[etpinard]

merging with #932