It depends what you're trying to show. Wind speed vs power for a single turbine is just physics. Weight all of the local wind speeds by the number of turbines on the grid, and you'd get a more precise national estimate. Based on OP's comments, I'd guess that a lot of Germany's wind power comes from Bremen.

OP has produced a bunch of 'some kind of marker for windy day' against 'power from various sources.' The set lets you tell that wind substitutes for petroleum, presumably because gas and oil plants are more dynamic than nuclear or coal. It seems like he's picked a city that's reasonably representative and accessible to humans, rather than a complicated formula that might be more predictive but not useful to readers. It gives the impression that there's got to be a pretty good wind blowing before you get much out of wind.

Might be helpful to have a histogram of wind speed at the bottom, so reader can get some sense of how often wind power is a significant factor.