the coutries are as follow :
united kingdom
germany
austria
belgium
netherlands
luxemburg
sweden
norway
denmark
finland
switzerland
Why these ones and not France, Spain, Greece, Portugal ? Their parties are as much social-democratic, or as less, as the British or the Swiss one.
I think the author decided to exclude Spain, Greece and Portugal because these countries had dictatorships until the mid-1970s, so it is impossible to have data from 1945 to 2015. And I think he decided to exclude France because there was a time when the communists were the biggest let-wing force, and so, the socialists (social-democratics) had a small share of the vote not because the left was weak, but because there was a bigger left-wing force.