HN either doesn't use Google Analytics or didn't check the box to make their traffic numbers public. That's why the graph you link to has a dotted line, rather than a solid one. The accuracy is much less certain in their case.

(Sarcasm noted :-) For the uninitiated, this was around the time comment scores were hidden. Only PG knows whether this had any real effect on traffic; and even if so, whether that's a good or bad thing is subjective (less poor quality traffic / comments is good, and since HN is not directly commercial, lower traffic is not necessarily a bad thing).

Loss of value to everyone else from lack of comment scores is another question entirely...

If we have that much evidence that having no visible comment scores reduces poor quality traffic, I think I may have just switched sides in that particular debate.

I'm not saying that is the case, but it could be a very interesting revelation.