This is a feature request to provide some extensions of the igraph_path_length_hist() functionality (possibly implemented as a separate function):
Why is this useful if we already have functions to compute the distance matrices (igraph_shortest_paths(), igraph_shortest_paths_dijkstra(), etc.)? Because igraph_path_length_hist() works even when the distance matrix is too large to keep in memory. On my computer I can run the histogramming function for a 40000-vertex graph, but I cannot run the distance matrix computation without running out of memory.
igraph_shortest_paths_dijkstra() could probably be used as a workaround until this is implemented properly - just run it with a single source vertex, for each of the vertices in your graph. This way you only have to keep a single row of the distance matrix in memory.