Another demonstration of a custom D3 bundle using Rollup. This demonstrates importing d3-transition, which modifies selection.prototype to define selection.transition:
import "d3-transition";
(There are other features in d3-transition, but they are not imported here because they are not used.) Note that you must also import selection from d3-selection; otherwise, Rollup doesn’t know that it needs to include the imported code from d3-transition. Rollup isn’t perfect at detecting which code needs to be included (and which code shouldn’t be included), so sometimes you need to give it a hand.