>>85725471The curved lines represent statistics. The line labeled 50 means average height or weight, while the 75 means "taller or heavier than 75% of the group" and the 25 means "lighter or shorter than 75% of the group". Pick a location on the curvy line and look at where it points on the x and y axis. For example, here I followed the "50" line (average) to where it intersects with 10 years old on the X axis, and by checking the same point on the Y axis, we can see that the average 10 year old girl in the US in the year 2000 was around 137cm tall.
