Calculating percentage Missing value for variables using one object

It looks to me that you should be using the table or the xtabs  
function. You have apparently already decided not to use NA for  
missing values, so the instances in which variable1 == "" you should  
get counts with those functions:

dft <- data.frame(var1 = sample(c("", "this", "that", "and"), 120,  
replace=TRUE),
                dt = sample( seq(as.Date("2006-01-01"),  
as.Date("2007-12-31"), by="months"),
                            120, replace=TRUE))

mo.tbl <- xtabs( ~var1+ dt, data=dft)      # the =="" entry is the  
first row

 > mo.tbl[1,]
2006-01-01 2006-02-01 2006-03-01 2006-04-01 2006-05-01 2006-06-01  
2006-07-01
          2          1          1          2          2           
3          1
2006-08-01 2006-09-01 2006-10-01 2006-11-01 2006-12-01 2007-01-01  
2007-02-01
          0          1          1          1          2           
1          2
2007-03-01 2007-04-01 2007-05-01 2007-06-01 2007-07-01 2007-08-01  
2007-09-01
          2          2          2          0          1           
3          4
2007-10-01 2007-11-01 2007-12-01
          1          3          2

x <- seq(as.Date("2006-01-01"), as.Date("2007-03-31"), by="months")
plot(mo.tbl[1,]~x)

Calculating percentage Missing value for variables using one object

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