How to create a dynamic variance calculation

Hi gurus! I need your help

I try to crate a Webi Report with a table showing the item and the sales for each item, across the three last years, I need to calculate the sales variance between each month of the year vs the total sales of the year. How can I create the table with the result?

year 2014, 2015, 2016 as filter

month jan,feb,mar,apr,may,jun...  as filter

for year 2014 and month jan

item     sale          %sales                                                                  proj     %sales_proj                         dif_vars   

1          12          (month_sales)/total_sales for the year for item 1      11          month_proj)/total_sales     %proj  -  %Sales

for year 2014 and month feb

item     sale          %sales                                                                 proj     %sales_proj                         dif_var

1          10          (sum(sales))/Total_sales for year                            9          (sum(proj)/Total_sales       %proj  -  % sales

eg. jan 2014

1          12          12/173                                                                      11          11/170                           6.47 -   6.9              

eg. feb 2014

1           10          22/173                                                                      9          20/170                                11.76 - 12.71