Thanks for your response. I managed to find the solution.
We had Case Number with Start and End Date.So first I created one reference date using Start and End Dates called reference date. Reference date is the date for which we are doing analysis.
New Cases for a particular month is counted by in Column 1:
count(distinct(if( (month([Reference Date]) = [Case Created Month]),[Case Number])))
Closed Cases in the Analysis Month is Counted by in Column 2:
count(distinct(if(month([Reference Date])=month([Case Completion Date]),[Case Number])))
Now we had to find the total Cases Trend per month using New and closed Cases.
So firstly I calculated the the difference of New and closed cases per month to find the total cases per month remaining using in Column 3:
after this, Trend of Total cases can be calculated by
=if(Column(1)=0 and Column(2)=0,null(),