Generating graphs such as found on Sedgewick's Algorithms book on the MST chapters
It appears to be a random geometric graph. Points are placed at random within an area, according to some distribution, and an edge is added between every pair of points whose distance is less than some threshold.

A common case is to distribute the points uniformly at random within the unit square and add edges between all pairs of vertices that are at distance at most&nbsp;$d$ from each other, for some constant&nbsp;$d$. In other cases, the threshold might depend on the number of vertices.