I’ve always argued that there is no “perfect” design—either the context changes its usefulness, or the design can be improved in some way. (Hence my philosophy of evolutional UX.) No matter how good you are, there’s always something to learn.
I’m not completely sold on this idea, but I like the approach. I think using a lightened and desaturated orange line would work better than a dashed line–the spaces between dashes create optical effects. I’d also label each line directly, instead of having a separate key.
It may possibly be overkill, but I also changed the data point markers: the NPP marker is square, the CO2 markers, both inverted and actual, are round.