Both lines use the range of 0-100. The BMI range is from 25-100 and the percentage is obviously from 0-100. They can both use the Y axis and be understood.  Where to start the BMI range is tricky. Realistically it should start at the most attractive BMI which would be around 18-20, but that's subjective. Underweight would show a decline as well, as you say, but that's not what the statistic stated. I thought I would keep it simple and start from 24.9 to 100BMI. I'm trying not to take it too seriously, though :)