Sankey hover text & styling

[alexcjohnson]

The initial release of sankey diagrams includes link.label which can be used to include extra text for each link that's only displayed on hover. Currently though, nodes have no such capability - their labels are displayed on the diagram and there is no way to display more or different information on hover.

While we're at it we should extend custom styling to sankey hover.

See https://github.com/plotly/plotly.js/pull/1582/files/8d021e0640a44da68a333426d77035ff67e27108..4d3ce9203fc6c2a0ffa21d9236708655b689e636#r115788926 for more info

[monfera]

@alexcjohnson could you (or I) split the item into two parts as I think @etpinard has been looking into the 2nd part already

[etpinard]

@monfera no that second part is new.

[antoinerg]

@alexcjohnson I think the current issue is addressed by the PR #3150 that you helped me put together a little while ago. Can we close this issue?

[antoinerg]

Styling was implemented in PR #3150

Custom content in hoverlabels will be supported via hovertemplate #3269 .

Closing in favor of #3269