[#14] implement overlay circle generation through point function
[#14] implement overlay node generation
[#14] add overlay bubble node translate
[#14] extract abstract bubble chart class
[#14] pull up radius calculation to abstract bubble chart class
[#14] implement radius calculation in bubble overlay chart
[#14] add radius transition to bubble overlay
[#14] refactor title and label function definition
[#14] pull up title and label rendering logic to abstract bubble chart
[#14] rename update title & label functions for better consistency
[#14] implement label rendering logic
[#14] fix duplicate label generation problem
[#14] register onclick handler for bubble overlay
[#14] implement title rendering logic on bubble overlay
[#14] implement color filling logic for bubble overlay
[#14] split bubble update logic from initialization logic
[#38] split dc.css into example.css + dc.css for better reusability
[#14] implement selection highlighting in bubble overlay chart
[#49] switch data table implementation from block element to html table
[#49] introduce colspan to data table group cell