You can use the XL Toolbox to change the plotting order of chart series. Move them to the front or back just like you do it with drawing objects elsewhere in Office.
In Excel 2007/2010, the commands are available from the “Chart Series” menu on the Ribbon. You need to select a chart series before using these functions.
In Excel 2003 and earlier, you can also use the menu commands, or simply right-click on the chart series that you want to promote or demote.