MrE,
I was able to duplicate your issue in my version of the sys (4.2 M1).
It appears that this option isn't yet available.
A recommendation would be to write a SAS stored process that completes this.
Included is an example of this code (I successfully tested in 9.2 and saw documentation that this option was available in 9.1.3.)
proc gchart data=sashelp.shoes;
hbar region / subgroup=subsidiary sumvar=stores inside=sum;
run;quit;
~ Angela Hall
http://sas-bi.blogspot.com