Jonathan:

The World Health Organization and UNICEF are in the process of
developing a project to collect data on healthy breastfed
infants/toddlers world-wide.  These data will then be used to develop
a new growth chart for children, which hopefully will be available
within the next few years.  Apparently, there are efforts also in the
USA to develop a only-USA based data set, as well as one in Europe.

For now, one can get an idea of what breastfed babies growth
curve looks like through small studies, such as those carried
out by K. Dewey, from the U. of CA at Davis.    See:

Dewey KG, Heinig MJ, et al.  Growth of breast-fed and formula-fed
infants from 0 to 18 months: The DARLING study.  Pediatrics Vol 89,
No. 6,  pp. 1035-1041.  1992.