Leave your current month field on the columns shelf, then add a calculation like;
IF month(date) = 1 then "January"
elseif month(date)=4 then "April"
Drag the calculation to the column shelf, the right click your original month blue pull and de-select show header.
Hi Tom, maybe that wasn't clear. I meant I still want all values to show, I just want to format the axis like that.
Nope, you were clear. Follow the steps above.