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From link above:
Anchors (blue 'x') can be configured using points=[[x0,y0],[x1,y1],...,[xn,yn]] in the style of vertices (via Ctrl/Cmd+E). The numbers are relative to the width and height of the vertex with values between 0 and 1.
Here is an example:
It's also possible to connect to anywhere in the shape by pressing the alt key while connecting:
To connect a new (unconnected) edge, select the edge and click on a shape in the sidebar:
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