add textposition property for Sankey plot's node labels

[banderlog]

Currently, there is no way to modify the position of node labels of a Sankey plot. Like putting them on nodes, for example.

How plotly.python cannot do (node labels are centered):

[ABSOD]

This is an important issue I would like to see addressed. I would like to move the label nodes so that they do not overlap the flows (as they currently do). Have not seen a workable solution to this issue.

[gvwilson]

Hi - we are trying to tidy up the stale issues and PRs in Plotly's public repositories. If someone would like to put together a PR to address this issue, we'd be very happy to review it. Thank you - @gvwilson