My minimal requirements:
free
theme customizable, including fg/bd colors, axises
can draw > 1 graphs
can show data on the graphs(tooltips, e.g.)
Flot: http://code.google.com/p/flot/ good and free
gRaphael: http://g.raphaeljs.com/ this one can show multiple y values for a given x value if there are more curves.
jQuery Visualize Plugin http://www.filamentgroup.com/lab/jquery_visualize_plugin_accessible_charts_graphs_from_tables_html5_canvas/ The data table is editable, impressive.
bluff: http://bluff.jcoglan.com/ can work with existing table data.
google chart: http://code.google.com/apis/visualization/documentation/gallery/annotatedtimeline.html can zoom in and out, nice.
flotr: http://solutoire.com/flotr/ can zoom in and out too
Canvas 3D: http://dragan.yourtree.org/code/canvas-3d-graph/ this one can draw 3D
jqPlot: http://www.jqplot.com/ can zoom in/out and candle chart as well