D3 or Data Driven Documents is a JavaScript library for manipulating document object model (DOM) elements based on the underlying data. In this sense, D3 is more than just a charting or visualization library. It gives you all the power of the DOM and the web standards around it such as HTML, SVG and CSS.
This graph shows the growth of San Diego Python since the group was rebooted in February 2012. Growth has been very steady and regular.