To change the stacking order for series in charts under Excel for Mac 2011:
- select the chart,
- select the series (easiest under Ribbon>Chart Layout>Current Selection),
- click Chart Layout>Format Selection or Menu>Format>Data Series …,
- on popup menu Format Data Series click Order, then click individual series and click Move Up or Move Down buttons to adjust the stacking order on the Axis for the subject series. This changes the order for the plot and for the legend, but may not change the order number in the Series formula.
I had a three series plot on the secondary axis, and the series I wanted on top was stuck on the bottom in defiance of the Move Up and Move Down buttons. It happened to be formatted as markers only. I inserted a line, and presto(!), I could change its order in the plot. Later I could remove the line and sometimes it could still be ordered, but sometimes not.