Tried to add age to the code so that bubbles would become transparent over time and eventually dissapear (and get removed from Circle_array so that they don't hog any more memory or get drawn), but I also changed the way that colors are handled and initialized (to an array rather than a string). Something broke.
A basic life-cycle which consists of slowly increasing transparency until the circle dissapears (and gets deleted from Circle_array). There are a couple of kinks to work out, however. First, which elements are being removed from the array appears to be buggy. Second, there is not always a smooth transition when a bubble is deleted - it's transparency isn't low enough when the delete happens. Third, it would be nice if the black circles around the bubbles were also increasingly transparent, but I couldn't get rgba to work when drawing the outlines.
Merged Alex's reorganization and optimizations - changed spacing in js code using jsfiddle.net
Nitpicky cleanup
Futzed with way bubbles fade out and changed them to a solid backgrpund for now - still needs lots of work, but it's better now.
I've gotten bubbles to dissapear in a bit more of a stable way and have their appearance the way I like them - but I'm still not happy with how stable deleting bubbles is (whenever a large number of deletions are performed, you encounter blinking) and the performance of the drawing function needs a lot of work.