yapp_plplot improvements needed

[teuben]

1. parsing the yapp= should be bettter enforced (e.g. yapp=png would leave a binary file for the png, if nothing after the comma, should assume some common name (yapp.png)

2. the default plot is not square

3. the configure options don't quite work (e.g. -lplplotd is reported)

[teuben]

some improvements in the code. Plot is square now, -lplplot is needed in makedefs, absence of %n in filename will trigger non-family of filenames.

triggered by the bismal state of pgplot as non-free (and thus absent in homebrew now)

[teuben]

also look at https://pg2plplot.sourceforge.net/