@data_null__ has provided an excellent solution. I am annotating my original code to show how I modified it to add the zero frequencies and showing the resulting plot. The key is the completetypes option in proc summary, which adds zero frequencies automatically. proc summary data=events nway completetypes;
class month year;
output out=_4plot;
run;
proc sgplot data=_4plot;
vline month / group=year markers response=_freq_;
yaxis label="Frequency";
run;
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