hovmoller question

[Teddy Allen]

Dear GrADS community,
I am a new GrADS user and am progressing quickly with the software thanks to the generosity of this group and the supporting documentation. However, I am currently stuck on one application that I just can't seem to figure out.
I would like to create a shaded Hovmoller plot that describes monthly average values along the y-axis (12 tics) and longitude along the x-axis. I am using the GPCP monthly dataset and can easily generate a plot describing all of the years of the dataset along the y-axis (29 years), but can't seem how to figure out how to average the dataset to display monthly climatologies.  Any suggestions on how to display monthly averages along the y-axis on a hovmoller plot?
Below is my script for the hovmoller plot describing all of the years along the y-axis. I am using GrADS NC version 1.8SL11 on a windows computer.

'reinit'
'sdfopen c:\testdata\netcdf\precip.mon.mean.nc'
'set grid off'
'set grads off'
'set gxout shaded'
'set t 253 348'
*time above displays GPCP years from 2000-2007
'set y 1'
'set lon 270 300'
'set clevs 0 .5 1 1.5 2 2.5 3 3.5 4 4.5 6 6.5 7 7.5 8 8.5 9'
'set grads off'
'set yaxis 253 348 4'
'set csmooth on'
'd ave(precip,lat=15,lat=17)'
'draw xlab longitude'
'draw ylab time'
'draw title Meridional Average (lat=15N-20N) of GPCP 2000-2007'
'cbarn'
'print'
'disable print'

Thank you very much for your help.
Sincerely,
teddy allen

To move quickly, go alone. To move far, go together. --- African Proverb