I don't know enough about voting patterns in Sweden, or the rest of the EU for that matter, to even begin to guess how Undecideds go.  In the US they typically fall within the general polling pattern of the precinct - local voting area - as glossed by the overall trend of socio-economic class since most US residences are segregated by race then class.  On the East Coast there are still, as I understand it, ethnic neighborhoods and that would fit between the two, above.

I agree the larger the number of Undecideds the closer this group with track with the overall population.  

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre