Possibly this SAS GCHART DOC link (and excerpt) may help:
http://support.sas.com/documentation/cdl/en/graphref/59607/HTML/default/gchart-bar.htm
With subgroups, PERCENT displays the percent contribution of each subgroup to the midpoint value of the bar, based on frequency. The PERCENT values for each subgroup total the percent contribution of the bar to the whole. For example, if the percent contribution of the whole bar is 60%, the PERCENT statistic for all the subgroups in that bar are 60% total. To calculate PERCENT based on the SUMVAR= variable, use the FREQ= and TYPE= options.
Scott Barry
SBBWorks, Inc.