Bubble charts can display sets of values as circles. They're usable for datasets with sizes in the range 10 through 100. They're particularly useful for visualizing values that differ by orders of magnitude and can replace pie charts in those situations.
As bubble charts are more complex and slightly less common, we're going to need a flexible library to draw them. The excellent D3 library (http://d3js.org/) is a great fit; it provides a set of tools, (the core data-driven DOM API plus the "pack" data layout) that enables the creation of bubble charts.
We're going to draw a bubble chart displaying the numbers of visitors coming to our website from referring websites.