Oh no, not this again!
It's a logarithmic scale and is explained on the guy's twitter feed why he uses it. But basically:
"In the initial outbreak phase, a virus like this spreads exponentially not arithmetically, i.e a log scale is the natural way to track the spread"
Also...
The graph would be unreadable on a linear scale:
"2) If we show case numbers on a linear scale, it would look like UK, US etc have little to worry about: they’re down here and Italy is way up there! You’d also be asking readers to calculate an exponential curve in their head to see if two countries are on the same trajectory ???"