Add some salt into Chris's code.

proc sgplot data=work.cumstats;
band x=i lower=pctnever upper=pctswitch/ fillattrs=graphdata2 transparency=0.5;
band x=i lower=0 upper=pctnever/ fillattrs=graphdata1 transparency=0.5;
series x=i y=pctswitch / y2axis lineattrs=graphdata2(thickness=2) legendlabel='Always' smoothconnect name='b';
series x=i y=pctnever / lineattrs=graphdata1(thickness=2) legendlabel='Never' smoothconnect name='a';
keylegend 'b' 'a'/position=topright location=inside across=1 noborder;
xaxis label='Iterations';
yaxis max=1.0 grid;
y2axis max=1.0;
run;