The stolen income is shown in the "Income from Other Sources" graph, not in the first graph. 

The third graph down, showing total credits earned might include it but I'm not sure.  This third graph is very revealing, because it shows just how much a Cruel is cheating by.  He's getting 4 times more income than you!  (Just compare the first few seconds for proof).

Embargo does work, and you are infact stealing income.  You can also look at his planets stats (hover over his planet), while you are stealing, and see the results at that moment.

Yeah, I thought you were asking about the end game graphs, (so we are talking about different things).

No, the stolen income doesn't show up in the ingame Credit Summary popup.  But you do recieve it. 

My S.W.A.G. is that it would be very time consuming to reprogram the credit summary to account for this one odd case and they never bothered to do it.

Generally it's believed that the AI is best with Vasari and worst with Advent.  If you managed a TEC cruel, you probably shouldn't have much extra difficulty against Advent.