Corrected with spelling mistakes, thanks for pointing that out.

So don't focus on the part before 2011, focus on the part after 2011 to till now. Simple as that
Your Google Trends idea has got me interested, so I had a quick look myself.
I knew Windows Phone was getting popular in India, but wow I never realised how much. Also, there's a lot of interest in Vietnam of all places.
Look at pretty much any country in the world (I looked at the EU5, BRIC, MINT, the USA and Canada), and "Nokia" has more interest than "Lumia", but interest in Nokia is either stable or decreasing, whereas interest in Lumia is stable or increasing. The Nokia brand is (and has been for some time) heading south. So eventually removing Nokia as a brand completely seems doable (and it has to be done, Nokia is still a separate company and still owns that branding). I expect "Nokia by Microsoft", where both company names have equal billing, is a stopgap for countries, like India, where the Nokia brand is still powerful, so that the Lumia brand has time to catch up.
I still think that in the US and Canada, MS should go for broke and switch totally to the Microsoft brand. The Nokia brand gets a lot less interest in the US and Canada than the Microsoft brand, and the Lumia brand is only a short way behind the Nokia brand, so those two countries are ready to have their brand focus switched.
I don't think comparing search term use of "Lumia" and "Surface" is going to be insightful, since one is a made up word as a brand, the other is an English dictionary word and therefore a generic term, there's going to be a few searches for that word that have nothing to do with a Microsoft table (you can see that in the graph, there's plenty of 'noise' from before the launch of the tablets). If you compare "Lumia" with "Microsoft Surface", you get a much better view, and in the US Lumia has still got more interest, in Canada it's pretty much neck and neck. Elsewhere in the world, interest is Lumia is way ahead.
So from the evidence, Nokia by Microsoft is a stop gap for the developing world, and out of the brands that MS own themselves, their order of value is Microsoft, Lumia, Surface.