Coldest European capital cities (January mean average temperature)

Yes, you are right, for the 1961-1990 period the average of Reykjavik was -0.55°C. But for 1981-2010, that is usually the new period considered for calculating averages, it's +0.05°C. Here is the official data of the Icelandic meteorological office: http://www.vedur.is/Medaltalstoflur-txt/Stod_001_Reykjavik.ManMedal.txt

I can anyway add it on the list, with the value of 0°C. And I will remove Luxembourg, that is actually slightly warmer.