While there are a variety of Javascript libraries available for creating visualizations, the most widely used is D3.js.
D3 stands for Data-Driven Documents, and at the core of D3 is a programming model in which users join data elements to document elements. These are often SVG elements, but D3 can really be used for any kind of data-driven DOM manipulation. I instruct my students on the basics of D3.js as part of the Visual Analytics course which I teach every fall.