How to draw a spline curve between 2 points with a control points with graphviz?

Question

I would like to create a spline curve between the blue point and the green point with the red point as a control point.

Graphviz documentation says :

1. splines attribute, of type bool or string, is valid on Graphs
2. splineType with the pattern : spline ( ';' spline )* is a valid type for pos attribute
3. pos attribute is valid on Edges and Nodes

I Tried this graph

graph G{     
layout ="neato" outputorder="edgesfirst" splines="true"
a[shape="point" pos="0,0!"        color="blue"]
b[shape="point" pos="1,0!"        color="green"]
c[shape="point" pos=" 0.5,0.5!"   color="red"]
a -- b [pos="e,1,0 s,0,0 0.5,0.5!"]   
}

then on Windows 10 PowerShell :

neato.exe -Tsvg .\spline.dot > .\spline.svg

with

neato - graphviz version 2.49.3 (20211023.0002)

result

What is the proper way of achieving this?

Thanks.

Answer 1

Close, but ...

• ! notation seems to only apply to nodes (not edges) (poorly documented, but see: https://graphviz.org/docs/attr-types/point/ and https://graphviz.org/docs/attrs/pin/)
• by default, pos value units are points (inch/72). Your values are ~ too small
• neato -n2 will use node & edge values you provide (https://graphviz.org/faq/#FaqDotWithCoords)
• all edges (not just curves) are defined as cubic B-splines and are defined by 1 + n*3 points (n is integer >=1) (https://graphviz.org/docs/attr-types/splineType/) (i.e. 4 or 7 or 11 or ...)

So:

graph G{
// default units for pos values are in points (72/inch)
// multiplied by 100 out of laziness 
// ! only applies to nodes, not edges
layout ="neato"
outputorder="edgesfirst"  // why???
splines="true"

a[shape="point" pos="0,0!"        color="blue"]
b[shape="point" pos="100,0!"        color="green"]
c[shape="point" pos="50,50!"   color="red"]
// "s" arrowhead must preceed "e" arrowhead,  so swaped them
// BUT, "--" says non-directed graph, NO arrowheads, so they are deleted
// also need 4, 7, 11, ... points NOT including arrowhead points
// so added points (just guesses)
a -- b [pos="0,0 30,66 70,66 100,0"]   
}

Gives:

Whew