A PPM file can contain only a single image, so for multi-page plots
PGPLOT creates additional files. If the supplied file name contains
the character #, the file name is derived by replacing this
character with the current page number. If the supplied file name does not
contain a #, PGPLOT appends an underscore and the page number for the
second and subsequent pages, e.g.,
PGBEG file name: File names used:
pgplot#.ppm pgplot1.ppm, pgplot2.ppm, pgplot3.ppm, ...
pgplot.ppm pgplot.ppm, pgplot.ppm_2, pgplot.ppm_3, ...
If the supplied file name is ``-'', the output is sent to the standard
output stream, so that it can, for example, be piped into a viewing
program. This will only work for single-page plots.