Don't know when the physical release was, but it should also be noted it was given away as  PS+ game on PS4 launch day, so that was a whole load of people getting it digitally for no extra cost.

Your chart is missing Sonic Forces and LA Noire as games that released on all 3 systems.

I don't know actually. But as I said in the OP, for pretty much every game there are special considerations. If it released from the beginning physically on the other consoles, it might be an explanation for the horrible number for Switch. But you shouldn't be too hopeful: all Sports game are bad on Switch, they all have less than 10% sales on Switch. The only other title going so bad is Doom, and this was a late port. FIFA, NBA, WWE all released at the same time at Switch and still do that badly. Seems the Sport-gamers aren't (yet?) in the Switch-userbase.

I only ask because the game has been in the Best Sellers list for Switch since it launched, and with the gap the majority of people probably bought it digitally compared to what the difference would be if physical and digital launched together.

I like the charts in the OP.

The ones that always stand out to me when I'm doing the sales are ones where the XO version leads, because it's so unusual (for obvious reasons). Just Dance 2017 is in the OP. Others include:

Rock Band 4 (but not Guitar Hero Live)
Garden Warfare 2
Dead Rising 4
Minecraft: Story Mode - Season Two
Fortnite's physical release
NHL 17 (but not 18)
R.B.I. Baseball 2016 (but not 17)
NASCAR Heat 2

It does not include (and these are interesting too):

Minecraft (mildly surprising given it had a holiday bundle)
Titanfall 2 (despite the first being exclusive. Although that clearly did ensure the gap was much smaller than usual for the sequel)
Rise of the Tomb Raider (despite the head-start. But the gap is much smaller than usual)

Regarding Sonic Mania - the sales in our database are for the special edition, which was available in limited quantities when the game launched digitally. A large scale physical release is due next month I think.