[gradsusr] Help multi-panel plots on a page

[Zilore Mumba]

As my earlier post had no heading, I am reposting the same in the hope that
it may be seen and I can get help.
I hope someone can find time to explain to me a few things on panel plots.
In the panles_demo.gs (snippet below),I do not understand the following:

1. for four plots on a page I loop through p=1 to p=4, why do we display t
(I want to display what I have in my file;
2. what is the global variable defined by _vpg.p in this script?
3. How do I put a title corresponding to the graphic validity time, on each
plot?
Help will be appreciated.
Zilore Mumba

  rc = gsfallow("on")
  if (args='')
    say 'Two arguments are required: the # of rows and # of columns'
    return
  else
    nrows = subwrd(args,1)
    ncols = subwrd(args,2)
  endif

  'use wrfout_d01_2013-07-27_00.ctl'
  'reset'
  panels(args)
  p = 1
  ptot = nrows * ncols
  'set mproj scaled'

* Loop through each panel and draw a plot
  while (p <= ptot)
    _vpg.p
    'set t 'p
    'set grads off'
    'd t'
    p = p + 1
  endwhile
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