[Request] Allow `customdata` attribute to be accessed by `hovertemplate` using `sankey`,

I am/was giving a workshop to an enterprise customer where they want the ability/flexibility to be able to pass in multiple sets of external metadata to customdata, that can be than used by hovertemplate for the sankey graph.

We tried using customdata from the top level data property, but were not able to access it. Currently hovertemplate can only access whatever is within it’s scope, e.g node or link. And, it can only take in one data set (list data structure). This could be helpful for other graph types as well.

CC: @alb-erandelcastillo

[antoinerg]

So perhaps, we could expose two customdata attributes: one inside the node container and one inside the link container - similar to how we currently expose node.hovertemplate and link.hovertemplate.

This proposal makes a whole lot of sense. 👍

[mtwichan]

If it’s not a complicated feature to implement it’d be great to have. This issue has come up for myself as a Dash/Plotly user many times, for users in the community forums (https://community.plot.ly/t/sankey-diagram-customize-tooltip/11487), users at a workshop I gave (for a paying customer - this issue), and now as a feature addition to a POC we’re trying to present to sway a client to get DE (https://github.com/plotly/dash-customer-success/issues/248).

I’d say it’s urgent in the sense that it could be useful for all the above reasons. But it’s not urgent in the sense that no one is paying for it. Up to you @antoinerg.