If you are ever likely to want to sort on the data, or possibly group by month and year, or year and quarter or similar you might consider adding another date variable with the appropriate default format.
dateonly = datepart(datetimevariable);
format dateonly mmddyy10. ;
Using the dateonly variable for the x-axis of a graph will have values sort in chronologically. If you use a character then you get a sort order like: 01/01/2015, 01/01/2016, 01/02/2015, 01/02/2016 which is not often the desired appearance.
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