Double grouping X axis - Year/month - visualization

You need to change the Concatenate label setting on the X-Axis.

Then ensure you have expanded the hierarchy on the visual

Some times you will also need to correct the sort order

Its a good modeling practice to have a seperate date dimension in your data model.  This table should include a rich set of date fields that you can use in your reporting.  One of the features would be to add an additional column that define the sort order of your months e.g. 1 = Jan etc...  You then need to assign this sort order field to the month label to ensure they are sorted in the correct way.

Hope this makes sense.