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I believe it's called "Sankey Diagram" as denoted in the dropdown menu on the upper top left.

Here is the original demo[1] from Mike Bostock, D3's author.

Though they have been around much longer than D3 has... https://en.wikipedia.org/wiki/Sankey_diagram

I believe it's called a Sankey diagram:

Sankey Diagrams. Useful for complex funnels!

We always just called them flow diagrams. I didn't know they had a specific name.

Where is the data from? It's a great overview of CO2 emissions, etc.