Sure, for this curve the r^2 isn’t as much of an issue, as you’re already talking about a derivative.
Which means you should not label your derivative curve “Observed”, as you are not directly observing it. “Derived” would be fine.
It’d be nice if you did include a reference line showing the actual observed data you’re basing your derivative on, but that’s not a big deal.
Of all of your graphs, this one depicts the most startling result, and appears to be the most worth investigating. However, derivative curves are tricky and ought not be approached without due care and attention, which I’m concerned about in your case given past issues with other graphs and what you’ve said about this one.