Hi Hanoch,
I'm afraid that your finding is the standard behaviour.
From the current month, 5 months before and 5 months after are displayed, hence 11 months.
This was so designed as the chart area is limited, With too huge numbers
and low screen resolutions, the look and feel might get spoiled.
However if you change the value of attribute 'NR_MONTHS_DDOWN_PERS_MAX'
of the class 'cl_crm_ppm_ui_utility' it should work.
You can set the value of 'NR_MONTHS_DDOWN_PERS_MAX' to 6 to
fetch total 12 months.
Hope my answer helps.
Shawn