Graphics library (low level)

• Raphael - http://raphaeljs.com/
• D3.js - http://d3js.org/
• Arbor (a graph viz lib) - http://arborjs.org/

Diagram Library

• JointJS - http://www.jointjs.com/
• jsPlumb - http://jsplumbtoolkit.com/jquery/demo.html
  • http://jsplumbtoolkit.com/jquery/flowchartConnectorsDemo.html

Misc Resources

• Building a UML editor in JavaScript - Part 1 
• Building a UML editor in JavaScript - Part 2 
• Building a UML editor in JavaScript - Part 3