Plots weight (in kilograms) based on the WHO growth charts for <= 2 years and the CDC growth charts for older than 2. The change in data source creates a slight "jog" in the graph at the 24-month point. It should be noted that these charts are prescriptive rather than descriptive; they reflect what the National Center for Health Statistics and the World Health Organization think the distribution of weights in children ought to be, rather than what it is (see the original paper, 2000 CDC Growth Charts for the United States: Methods and Development, page 16, and the discussion from the CDC about changing to the WHO tables). That's why we get otherwise meaningless statements about 15% of children being over the 95th percentile; the percentile lines reflect the distribution of weight from an "ideal" "healthy" population.
The weight distribution for children with Down Syndrome is from Health Supervision for Children With Down Syndrome, PEDIATRICS Vol. 107 No. 2 February 2001, pp. 442-449.
Sex is set with
sex=female. If neither is specified, then the percentiles are
down=yes to get the Down Syndrome graphs.
The 3 percentile line is actually 2.3% and the 97 percentile line is actually 97.7% for age <= 2 years; that's just the way the WHO published it.