How to include categories for which there are no values in an ssrs chart
The best solution I've found to this type of issue is to create a date dimension table that shows various information for dates- in this case it could simply be date and month, but other implementations I've seen include whether or not a particular day is a holiday or a day in which employees will work.

Then, apply the date criteria to this dimension table and left join the results onto it-

CREATE TABLE date_dim (Date_eval, Month)

...

SELECT  [t.DataPoint], [dd.Month]
FROM   date_dim dd
LEFT JOIN Table t 
   ON t.[When] = dd.Date_Eval
WHERE
    Date_eval &gt;= DATEADD(MONTH, -7, @CurrentMonth)
    Date_eval &lt; @CurrentMonth


The downside of this approach is that you have to populate a table with data and that if you use a date dimension attribute that can change, such as whether or not employees work a certain day, the table is difficult to maintain automatically.

The upside is that you have a framework to build reports like this on, and one that can be reused as often as you need it.