How to create date filed from monthyear field and create the rolling 12 momths in line chart?

[daisy1438]

Hi

I have fields like  monthyear  and periodmonth.

data like             APR-2016            APR

                          APR-2017            AUG

                          APR-2018             DEC

                      AUG-2016               FEB

                          DEC-2016          JAN

                       DEC-2017             JUL

                      FEB-2017              JUN

                     JUL-2016               JUNE

                        JUL-2017            MAR

                       MAR-2016           MAY

                        MAR-2017          NOV

                      NOV-2007           OCT

                     NOV-2018              SEP

                    OCT-2007          

                     SEP-2007

                     SEP-2016

my requirement is creating line chart  with rolling 12 months sales  .and using above two files how to create the date field.

please share the expression for rolling 12 months and date creation.

Thanks