Fonte Como alterar a ordem da legenda em um gráfico do Excel
Right-click on one of the names listed on your legend.
Click on the "Select Data" option from the list that appears.
Click on the entry you want to move in the "Legend Entries (Series)" box.
Click the "Up" or "Down" arrows in the box to change the position of the legend. Click "OK" when finished to save your changes.