Add Sankey depth labels

[JoeVanGundy]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

We need to be able to give the user additional context about each level of the sankey plot.
Currently, there doesn't seem to be anyway of doing this or even adding text to the sankey plot.

For example:
First Product----Second Product (10.9% of previous)----Third Product (10.0% of previous)

What does the proposed API look like?

The sankey can have an axis, which then could have an axis label.
A simpler solution would be that the Sankey plot could have text at an x and y coordinate. Then I could calculate where each label should go based on the number of levels to the sankey.

[ckifer]

The ability to add column labels seems reasonable - labeling as feature request

[FrederikPM]

I would just like to chime in and say that we are also in need of this feature.