jqplot is a best graph free jquery library. It provides numerous of chart options. jplot graphs are not only customizable, they can also be expanded. It enable users to zoom, add shadows and even drag & drop in graphs/charts.
Flot is another jquery chart library which is simple and nice. Its very simple to create graph and provide functionality to change all settings of the graph. It provide amazing features such as zooming, data points interaction, turning on/off series and more.