Question: how do we draw a vertical band at the maximum value of a single-stream streamgraph in d3js that respects the bounds of curve?
Imagine this is the original streamgraph:
The question is focused on how we create something like this:

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The following solution uses @GerardoFurtado's initial idea, but adds a clipPath to respect the bounds of the curve for any bandwidth:
https://beta.observablehq.com/@blehman/untitled/2