labels are associated with ticks so you have two choices:
- Add ticks at six-month intervals and label the mid-year ones with empty labels for the beginning of year locations, or
- Don't use tick labels at all but write the months text with text at the six-month x-axis locations.
ADDENDUM
Oh, there's a third that comes to mind -- add a second x-axis on top of the existing that has ticks at the mid-year position and use labels on it and clear the first.
