ts-plot-* script fail catastrophically when the output path is specified as a relative path

The plot scripts assume the output path is an absolute path (this also applies to the input path if the output is not explicitly given). If a relative path is provided, the scripts switch CWD to where the input is, which invalidates the output path, no validation of the gnuplot output file name is made, which causes gnuplot to direct its (binary) output to the console, with all sorts of unpleasant side effects.

The requirement to specify absolute input and output path should be explicitly documented (currently it is not), and the gnuplot output file names should be validated before invoking gnuplot.