Add zero frequency with VLINE

[aknez]

I have a data set of events that occur over time. Each observation is an event with a date and I create two variables from the date: month and year. I use VLINE to plot the frequency of events that occur each month over several years.

I love how easy this is to implement in SGPLOT with VLINE but I would like to add one thing: in Dec 2018 there were no events (see blue line). Is there a way to have VLINE plot a zero for this month?

proc sgplot data=classes;
 vline month / group=year markers;
run; 

[data_null__ · Accepted solution]

At the urging of @novinosrin I am adding notes.

My first thought and was to use the FREQ= option but I found that values with 0 are discarded.  So I ended with the trick of plotting the sum.

The first step is something from nothing  COMPLETETYPES.  This creates obs with zero freq for AGE=16 sex=F.

Plotting the SUM of _FREQ_ is straight forward but you don't get a nice Y axis label but that can be fixed. 

yaxis label='Frequency' values=(0 to 3);

ODS TRACE and ODS OUTPUT are not really needed but I want to see the data that SPLOT used.

proc summary data=sashelp.class nway missing completetypes;
   class age sex;
   output out=_4plot;
   run;
proc print;
   run; 
   
ods trace on;
ods output SGPlot=SGPlot;
proc sgplot data=_4plot;
   vline age / group=sex response=_freq_ stat=sum markers;
   run;
ods trace off;
proc print;
   run;

[novinosrin]

Guru @data_null__  Needless to say as usual yours has to be great. May i request you to add a couple of comments that addressed the solution appropriately so that people can diligently take notes? Only when you have a moment to spare. I understand!

[aknez]

@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;