sfpldb and splas are utilities for debugging Vplot files by converting them to the plain text (ASCII) form. pldb (plot debugger) converts a Vplot file to a text form, plas (plot assembler) converts the text form back to the Vplot format.
Suppose, for example, that your Vplot file has a typo in the label which spells Dept instead of Depth. Here is a Unix one-liner for fixing the label:
Here the Unix line editor sed gets sandwiched between sfpldb and sfplas.
These programs were initinally developed by Vplot’s original author, Joe Dellinger.