Actually true, making allowances for that graph in panel 3 being simplified to fit.
But not that hard to figure out, so it’s not really a CIDU for an audience as clever as ours. Eventually it will be one of those phrases where the original meaning of the term becomes ever more obscure, e.g. “not worth a plug nickel”.
Here’s how it was done in 1943:
Comics I Don't Understand 
I don’t know why it’s meant in the comic, but IMO I think the renewed interest in tap is directly related to social media and pandemic isolation.
Look up “tap” on TikTok. :)
Are we speculating that the modern usage is mostly metaphorical, referring to tap dancing around a topic?
Tap is still taught to kids in dance classes.
By the way, the graph in the comic wasn’t simplified; it was smoothed, which Google Ngrams has set to 3 by default.
I tried adding “tap dance around” to the Ngram but it definitely is not enough to account for the increase. I switched to “tap dancing” and there’s still a gentle increase since the late 70s but it’s not nearly as dramatic as “tap dance”.
Tap is still taught; my granddaughter takes it. But her class was 2 minutes out of a 2.5 hour dance recital we were at last spring. It’s not popular at the moment.
So…we still tap dance, just not in ‘tap dancing halls’.
Like still we have jokes, just not in this comic.
Regardless, what is the punch line supposed to be in the comic? That the blue shirt guy doesn’t know how to read a graph? Along with the decline and disappearance of tap dancing halls, our critical faculties and concomitant ability to understand statistical representations have also abandoned us? What a world, what a world.
The graph has no numeric values on it, so is useless as a map with no indication of scale.
Harry Farrell, an old newspaper reporter, described being assigned to interview a boring public health official. The official mentioned there had been two reported cases of VD in the past quarter, and one in quarter before. Farrell’s headline was COUNTY VD RATES DOUBLE IN THREE MONTHS.
The joke is simply that at a sufficient scale almost anything will have variance that seem significant. The earth is relatively smoother than a billiard ball.
I’m just a touch surprised there were no comments about how the guy was able to pull up a google ngram but unable to use Google to find a tap dance hall.
One current instance of tap dancing is Irish dancing, particularly as danced by the Gardiner brothers. Search YouTube shorts for them, they’re pretty good, although it’s not strictly classical jigging.