Plotting $f$ and $g$ together is unlikely to lead to intuition about the GMVT; they're just two arbitrary curves, whereas the value from that diagram of the MVT comes from the fact that the yellow line is constructed to give intuition. The art of drawing intuitive figures is largely about figuring out what you need to construct in addition to the raw givens. So I'm going to suggest, not a ‘transmogrification’ of that particular construction, but a different construction that targets the general case.
Consider $f_1(x) = f(x) - k \cdot g(x)$, where $k = \frac{f(b) - f(a)}{g(b) - g(a)}$. If $f_1'(c) = 0$, then $f'(c) = k \cdot g'(c)$, and so $c$ satisfies the GMVT condition. It's easy to see that $f_1(a) = f_1(b)$, so an illustration of looking for some $c$ where $f_1'(c) = 0$ on a plot of $f_1$ over $[a,b]$ is just the figure for Rolle's theorem, as it appears in that large screenshot.
(P.S. Please avoid making screenshots the bulk of your post in future.)
r~~
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2021-02-08T19:57:58Z (almost 4 years ago)