From the looks of it that's been generated by `circo`, which is part of GraphViz and describes itself as a “filter for circular layout of graphs” and ci

From the looks of it that's been generated by `circo`, which is part of GraphViz and describes itself as a “filter for circular layout of graphs” and circo draws graphs using a circular layout (see Six and Tollis, GD '99 and ALENEX '99, and Kaufmann and Wiese, GD '02.) The tool identifies biconnected components and draws the nodes of the component on a cir‐ cle. The block‐cutpoint tree is then laid out using a recursive radial algorithm. Edge crossings within a circle are minimized by placing as many edges on the circle's perimeter as possible. In particular, if the component is outerplanar, the component will have a planar layout. If a node belongs to multiple non‐trivial biconnected components, the layout puts the node in one of them. By default, this is the first non‐ trivial component found in the search from the root component.