Reference : http://www.maani.us/xml_charts/index.php?menu=Reference
chart_type = bar, circle, line, column (default), stacked column, floating column, 3d column, image column, stacked 3d column, parallel 3d column, pie, 3d pie, image pie, stacked bar, floating bar, area, stacked area, 3d area, stacked 3d area, candlestick, scatter, polar, bubble