Have a look at the attachment. By, now, when you select a series, you can bring
it forward/backward (cf screenshot). This is ok. What could also be implemented
is the ability in a diagram to arrange all the series at once following a given
criteria.

For exemple, all the series could be sorted by category name (alphabetically),
by value... This could be inserted in the UI when in the Arrangement sub-menu.
When a user 

- bring forward
- bring backward
----------------
- sort alphabetically
- sort by value
- special sort...

The "special sort" could open a dialog and give to the user the choice in which
to order the series. Some more complex examples :

- sort the series given the mean of the sum of the series values.
- sort the series given the max value of the series
- ...

This dialog could tak inspiration from the data pilot dialogs.