The request is for the users to be able to add/remove series just like
they do the categories.
The categories have a checkable listbox (the field listbox) that you can
select and deselect items in.
The series data does not seem to have such a checkable listbox. I'm
completely new to OWC Charts and I am not sure that you can remove
series from the legend and add it back.
If you r-click on the legend, you can delete an entry using the Commands
and Options wizard. Once the wizard opens, use the drop-down to select
an item you see in the legend. Then, click the X to the top left of the
drop-down listbox you used to select your legend item.
This removes the item from the legend, but I see no way to add it
back again.