Because you're plotting what's called a "powercurve". It's a standard figure in the wind industry, because wind is nearly always measured in m/s or knots.

I've worked with wind turbines for 11 years and made hundreds of powercurves. Never seen anyone use km/h before.

Wind turbines derate earliest from 20 m/s and cut out around 30-40 m/s. On this graph it drops off at 14 m/s, so it doesn't fit.

Also I hate seeing powercurves in km/h. Nobody does that in the wind industry.