fio2gnuplot: Fixing plotting issues on shorten files

fio2gnuplot: Fixing plotting issues on shorten files

fio2gnuplot was mixing files when some got shorten.

The position inside the array was used to remember what file was read
leading to mistakes once some values got ignored.

As a fix, we shall count the index for every single line we read and add
to the time & perf we keep. While writing data to the temporary files,
the extraction of the index insure we write to the proper file.

Before this patch, we observed at the end of the traces some stupid artefacts like loops... was
looking like a 4years old kid drawing...

That's now fixed.