Reordering Chart Data Series

Right-click any series on the chart. In the "Format Data Series" dialog, there is a "Series Order" tab, in which you can move series up and down. I find this much easier than fiddling with the last argument of the series formula.

This is in Excel 2003 in Windows. There is a similar dialog in Excel 2011 for Mac:

Select a series and look in the formula bar. The last argument is the plot order of the series. You can edit this formula just like any other, right in the formula bar.

For example, select series 4, then change the 4 to a 3.