Thanks to Mike Bostock and his remarkable D3 library for the heavy lifting here. All I've added is the ability for a user to enter and edit the data to make it easy for people to make their own visualisations for their own use.
Mathematicians may argue that using circles to represent data is poor, for various reasons (e.g. is it the area or the height; that humans can't easily measure things by area etc.), but they are missing a couple of things:
Enter your data in the form below and you can watch the visualisation building as you enter your data. First of all, clear the sample data by clicking the “Clear all data” button at the bottom of the Chart Data section.