Data Driven Documents – is a visualisation library for Javascript used by The New York Times, The Financial Times, The Guardian and loads of other people too.
Most software for producing data visualisations takes the approach of providing the user with a bunch of general types from which to select (bar, pie, line etc.) and then allowing for limited customisation – this approach tends to result in pretty basic charts that are not quite right for the purpose the user intended and which can end up with a whole stack of configuration data. (that said, highcharts is a pretty good example of this approach)
D3 provides a more flexible and general set of tools for visualising data, though if you want you can still make a pie chart.
In the talk I outline a general pattern for thinking about data visualisations that fits with the way D3 works, a kind of dataviz MVC .
Useful links for learning D3