Subplots multiple traces

[PierreSnell]

The goal would be to allow the same behaviour as https://stackoverflow.com/questions/46750462/subplot-with-plotly-with-multiple-traces which is to be able to create subplots and each of those subplots contains multiple traces.

[andrei-ng]

@PierreSnell Unless I am missing something, this was/is already supported.
I have added an example in PR #313