Is your feature request related to a problem? Please describe.
Right now, all nodes of a certain depth are justified to fill the vertical height. It's useful to be able to have nodes "stick" to the top when representing sequential processes where vertical position can reinforce the natural reading order.

Describe the solution you'd like
Ability to pass in prop for vertical alignment.

Describe alternatives you've considered
Currently unable to control vertical alignment with recharts Sankey.

Additional context
An example of vertical top alignment in analytics tools like Amplitude: