Set the category field to "=Month(Fields!closedate.Value)
", or something similar that will net the results you need. Right now, even though you're grouping by month, you're still telling SSRS to use the atomic data for your X axis, so that's what it's doing.
It may make your task simpler to just add a Calculated Field to your dataset - open the dataset properties window, go to the fields tab, and click "add". Set that field to your month value, use it for grouping and your X axis.