jqPlot produces beautiful line, bar and pie charts with many features. jqPlot's Strongest Feature is its Pluggability! Computation and drawing of lines, axes, shadows even the grid itself is handled by pluggable "renderers".
Not only are the plot elements customizable, plugins can expand functionality of the plot too! There are plenty of hooks into the core jqPlot code allowing for custom event handlers, creation of new plot types, adding canvases to the plot, and more!