These are a few questions already asked through e-mails. It should be organized by themes.

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- '''How can I plot the land/sea mask on the SST graph like you have on
yours - is this defined in grid_config.def ?'''

- No, all these global plotting options are set in plt_def.pro (which is next to post_it.pro).
For the continent one, you need:
{{{
   cont_fill = 1         ; continent fill for atmosphere grids (1/0)
   cont_real = 1         ; draw real continents (1)
}}}

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- '''Is there a way of altering the colour bar ? The colour bar on the
attached Dec1997_MSL (mean sea-level pressure) graph appears to be
erroneous, with no values shown.'''

- The colors are defined via 'color palettes' in the /config/palettes directory. The name of these is built from the variable name. There is already a mean sea level pressure one and what you just need to to is add the following line to the 3 files (in config) fld_glo.iso.def (global isolignes and mult/add factors), fld_glo_mmx.def (min/max) and fld_zoo_iso.def (same as glo_iso but for zoom plots)
{{{
MSL @slp
}}}
This will tell post-it to use the 'slp' contours and color settings which for instance in fld_glo_iso.def are :
{{{
slp         3     3     2     4    1016     2     4  0.01  0. hPa
}}}
The last 3 are for unit change from Pa to hPa (ax+b=001x+0), the first two (3,3) are not used and the next five set contour level in B&W plots (i.e when color palette are not used - see pal_type in plt_def.pro)

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- '''How can I use Post_It to plot pressure level data at a particular pressure? The ggap files contain variable values at each pressure value and I would like to plot the values at one pressure only.'''

- First make sure the field to read is 4D (use ncwa to get rid of spurious dimensions) and then in plt_def.pro, use vert_mean to choose the right level 
{{{
; vertical average for 3D fields
   vert_type = 'z'          ;  'z' for depth/altitude or 'level' or '0' for nothing
   vert_mean = [xxx, yyy]   ; [depth1,depth2] or [level1,level2] in C notation 0-jpk-1 
}}}
This feature has not been used in a while for 3D atmosphere fields and may need some small fixing.