Postscript.
What you can also do with a graph like that, is take the speeds at which the maxima occur (in this case 97 knots for a 20kn headwind), about 67 knots for a 10 kn headwind and probably around 45 knots for still air, and plot two more graphs from those.
- Best range speed against head or tailwind component.
- Range (or range per fixed fuel quantity) against head or tailwind component.
Which can be useful for some aircraft types - sort of thing I used to do e.g. for a military patrol aircraft like the Nimrod, or just for emergency "how the hell am I going to get home" procedures.
G