The text at the bottom says DXY. It's a return when measured in foreign currency (mostly EUR)

Yup, but we can reasonably assume the others are based on USD valuations.

I did realize I got the "flip upside down" wrong, it would be the reciprocal instead, which for low percentages is nearly identical (1 / 1.1 = 0.909) whereas flip vertically gives (1 - (1.1 - 1) = 0.9)

Yup, it makes no sense to include it on this graph. Text at the bottom says it is the DXY index. So, it is being measured against a mixture of foreign currencies.

One way to make it almost kinda make sense would be to flip it upside-down and label it "foreign currency"

Pretty graph! However, including the DXY green line for dollars doesn't make any sense. Presumably all of the other assets are being measured in terms of dollars. DXY is an index comparing the strength of the dollar against other currencies, naturally it is not measured in dollars, so it doesn't belong on this graph.

To see why it doesn't make sense to use a percentage derived from a different unit, imagine if the rate of return of the dollar line was instead measured in terms of its value in bitcoin. This would make the dollar appear to be an amazing store of value in 2022!

Including a dollar line would make sense if the chart was of inflation-adjusted returns. In that case, though, the green line would be entirely below 0%, and the other lines would also be shifted downwards.