Length is not equal to height—take a look at this example:

This 2-year old child's length (91.1 cm) was plotted on a growth chart for 2 to 20-year olds – stature-for-age at the 90th percentile (black dot).

When her height (measured on the same day) was plotted, stature-for-age was at the 75th percentile (blue dot).

Therefore, if a length measurement is taken (e.g., because an older child is unable to stand) it should be noted on his growth chart.