Hello!

Whenever I try to watch the power graph, only me and two-three other civs appear in the graph. How do I make all the other civs appear?

I'm sure the answer is embarassingly simple, but please, have patience with me. :)

you have to have enough EPs more than your opponent

Look at the Espionage screen. For your enemies to appear on the graphs, you need to have enough espionage points against them to equal or exceed the figure shown against "see demographics". But it's best when you have enough to "investigate cities", as then you can open their city screens as you can your own (but of course you can't muck about with what they're doing, which is a bit of a shame).