If it was constant, I'd agree, but the natural temperature was always changing and whatever it would have been today will also be changing. So you'd be graphing

If it was constant, I'd agree, but the natural temperature was always changing and whatever it would have been today will also be changing. So you'd be graphing a different quantity, f(t) = temperature(t) - natural_temperature(t). From such a graph, you wouldn't be able to tell if a change was a change in the actual temperature or just a change in the predicted natural temperature. A predicted natural cooling would look the same as a human-caused warming.