Hi:
For a numeric variable (such as month -- if your month is a numeric variable), PROC GCHART determines the midpoints as though month were a continuous variable -- which may not be what you want. In the documentation of the PIE statement:
http://support.sas.com/documentation/cdl/en/graphref/63022/HTML/default/viewer.htm#gchart-pie.htm
...you will find several options, such as the DISCRETE option and/or the MIDPOINTS and/or the LEVELS option, which may help you.
There is a separate forum for posting SAS/GRAPH and ODS Graphics questions, which is monitored by the SAS/GRAPH folks. In the future, you might want to post SAS/GRAPH questions there.
cynthia