It looks like it is more canvas than svg, yes? Was there a particular canvas library that you used? I ask because I'm working on a side project that feels like it mostly pulls me in an svg direction, so I'm been prototyping with dagrejs and cytoscape.js, but it also has elements of creating (or dragging, like yours) new elements into a graph and connecting them, so I'm regularly torn when deciding on the basic architecture of it.
I'm not using canvas. SVG is used for the connections, the rest is HTML. I'm not using any particular library for the diagrams, since I couldn't find anything suiting my purpose unfortunately.
I would definitely like to improve the look of the wires.
