In a stylesheet for relevant edges do

<add as="verticalAlign" value="top" />
<add as="verticalLabelPosition" value="bottom" />

Or do it inline when inserting an edge

graph.insertEdge(..., 'defaultEdge;verticalAlign=top;verticalLabelPosition=bottom');

"defaultEdge" is there to keep the rest of default styles, not override them completely.

This positions label bounds on the bottom of an edge, and positions labels at the top of their bounds.