求助高人：画图的问题
有下列程序：
data a1;         
  input g $ y $ a b;
  datalines;
g1 y9 0.1 11
g1 y9 0.2 12
g2 y8 0.2 11
g2 y8 0.3 12
g3 y7 0.3 11
g3 y7 0.4 12
g4 y6 0.4 11
g4 y6 0.5 12
g5 y5 0.5 11
g5 y5 0.6 12
;
run;
proc sort data=a1 out=a2;
by y;
run;   
proc sql noprint;
select cats("'",y,"'") into :zhu separated by " " from a2;
quit;
goptions reset=all ;
axis1 label=none style=0 order=(&zhu);
proc gchart data=a1;
vbar b
/sumvar=a subgroup=y group=g discrete vref=0 raxis=axis1;
run;
quit;

我想按照Y的排列顺序画图，也就是g5在图的左边，，，g1在图的右边。
运行程序后报错：
WARNING: The character ORDER= list on axis statement 1 was ignored because the axis labeled a SUM represents a
         numeric variable.
求教高人如何实现，谢谢！