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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:
points=[[0.25,0],[0.5,0],[0.75,0],[0,0.25],[0,0.5],[1,0.5],[0,0.75],[0.25,1],[0.5,1],[0.75,1],[1,0.1],[1,0.2],[1,0.3],[1,0.4],[1,0.5],[1,0.6],[1,0.7],[1,0.8],[1,0.9]]
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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