Re: google maps and data visualzation

You can change the icons of markers to anything you want. So, this
includes changing their color. Here's some icons that people have
already made:
http://mapki.com/index.php?title=Icon_Image_Sets

I'm not exactly sure what you mean by "Can I merge each individual
address into groups of
some kind?" You'll have to explain.

The API doesn't provide any method for group of points, but lots of
discussion about this has gone on. Just search this group.