Summation with Cacti 0.8.7i - How to configure

[MDP]

Hi All
I have been using cacti for a while now but am in no way an authority - I can add graphs using the GUI and that's about my limit.
I have a need to be able to add some graph sources together to create an accurate bandwidth total across multiple interfaces that are being captured individually, I believe this is known as summation.
Is this possible with 0.8.7i?
If so, can anybody help me out with how to achieve this? Preferably with the GUI, I have seen some guides for older versions so presume it is possible but cannot see how to achieve it.
Newbie apologies if this has been covered a thousand times before, but the posts I could find are not relevant to 0.8.7i as the menus are so different.
Many thanks for any advice
Mike

[MDP]

Does nobody use summation?