@simontatham how complex of a graph, and is the interactive portion a hard requirement? I'm slightly biased towards TiKZ, as it lets me define nodes, edges, and their attributes declaratively. It's not exactly interactive, but if you have a decent LaTeX editor then at least you can see the result of your graph declaration quickly, and I find that it minimizes the amount of tinkering I need to do while constructing charts. There are also markdown-targeted libraries like Mermaid that claim to do the same. I don't particularly like them, but I'd be negligent if I didn't at least mention them.
For truly interactive tools you're probably going to need to dip into commercial solutions. Visio is the classic offering for Microsoft systems. OSX's equivalent is OmniGraffle. And the online equivalent is Lucidchart.