Graphviz
The tool that draws graphs for you. You describe nodes and edges in the DOT language and a layout engine works out where everything goes. Developed at AT&T Bell Labs from the early 1990s.
The syntax is hard to beat for brevity: digraph { a -> b -> c; a -> c } is a complete flow chart. The layout engines pick a strategy — dot for hierarchical directed graphs, neato and fdp for spring-model placement of undirected graphs, circo for circular topologies. Output in SVG, PNG or PDF. Because both the input language and the algorithms have been stable for decades, Graphviz is used as a backend everywhere: Doxygen call graphs, terraform graph, the pprof profiler, dependency diagrams in package managers. The downside is that you don't control the layout in detail — the engine decides, and stubbornness with rank and constraints only helps so much.