A column with one dominant source and several minor ones is common in traffic and funnel reports (see issue 87): a flow of a few hundred next to flows under one. Without a minimum, those small nodes’ bars round down to a fraction of a pixel and effectively disappear. nodeMinSize stretches the drawn bar of Referral, Social and Email to at least the requested size, centered on each node’s real position — the underlying flows keep their exact proportions and connection points, so a flow of 0.9 is still a hairline inside its now-visible node:
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The node is visible now; the flow itself is not. If a 0.9-unit flow needs to be visible too, the data has to be clamped to a minimum before charting — that’s a data decision, not something nodeMinSize can do, since it only ever changes how a node’s bar is drawn, not what any flow is worth. See Node Min Size in Usage for the full semantics.