You can format the given date with the date function like below.
Date(givenDate,'MMM-YYYY') returns any date in MMM-YYYY format.
But remember it is just the formatting of the date.
Lets say if you have two dates for the same month as 29/04/2012 and 30/04/2012.
For both of the dates, the above formatting function returns Apr-2012. But still the original values are not changed.
So if you use this as a dimension in a chart, it will show 'n' number of same values 'Apr-2012' as it is available in the data. In this case, you can see two Apr-2012s in the chart.
If you feel any problem with this solution, there is always another way of doing it.
Date(MakeDate(Year(givenDate),Month(givenDate),1),'MMM-YYYY') AS anotherDateColumn.
You can use this to create new column 'anotherDateColumn'.
So you will get the table like the below.
29/04/2012 Apr-2012 (Original Value:01/04/2012)
30/04/2012 Apr-2012 (Original Value:01/04/2012)