I don't know if I can be of any help but here is how the chart appears to me:
I'd have thought they calculate % monthly ROI on starting price 1 month ago compared to current price, and then show the % gain during that period as the ROI.
I also think that''s the case. It seems to me that the current price of BTC is about 4 percent higher than the price from a month ago, hence ROI~=104%.
you'll also see all the other coins with those high figures, which I completely don't get.
Well the other coins have higher ROI's because a month ago they have been dumped much more than bitcoin. That's why their relative growth seems higher.
It's like that, when the market is in its bearish phase, only bitcoin manages to hold, all the others go down considerably more.