Frameworks for developing Metro Map drawing software?

Question

I'm looking to build a tool to draw simple Metro/Tube/Underground/Urban transit style maps. The concept being that there's a drag and drop diagram editor and a code window. Change the code by adding a station - the diagram changes. Change the diagram by dragging and dropping in a new station - the code changes.

A bit like a 2-way www.websequencediagrams.com

My question is what open source (or otherwise) libraries out there please that could assist on both fronts, primarily the UI of icons, a canvas, config boxes for the objects, but also on the syntax checking and highlighting and parsing of the diagram description language.

Thanks