I was already impressed by the accuracy of the SteamSpy polling algo when it was right on the ball regarding sales for Planet Coaster. I just noted that Paradox just released sales numbers for Hearts of Iron IV:

"Paradox Development Studio is thrilled to announce that Hearts of Iron IV, its World War II themed strategy wargame, has sold 500,000 copies worldwide since its release in June 2016."

SteamSpy reports 498,696 owners of Hearts of Iron IV.

I highly recommend anyone using SteamSpy for any professional or investment purpose to read about their methodology here.