I have the same problem. I Gnuplot 5.4 patchlevel 3 installed also get the same error:

Gnuplot not found or not usable, using plotters backend
`gnuplot --version` returned an unparseable version string: 

Still same issue error with gnuplot 5.5 patchlevel 0

Hello! Thanks for the bug report.

I haven't been able to reproduce this issue with gnuplot 5.4 patchlevel 4.

For those of you having this issue, can you check if gnuplot is maybe printing its output to stderr?

Sorry I misread, whe nrunning cargo bench it does not output to the terminal, I get an error of it being unparsable

`gnuplot --version` returned an unparseable version string: gnuplot 5.5 patchlevel 0

Well, what I wanted to know is whether gnuplot --version prints its version to stderr or stdout.

Works for me, Windows 11 and gnuplot 5.4 patchlevel 6. Likely not helpful to those with the problem, but it is a reference data point.

win10 gnuplot 5.4 patchlevel 6,
another data point:

1. make sure gnuplot/bin is in PATH
2. restart vscode
3. use cargo bench -- --plotting-backend gnuplot
4. returns "gnuplot --version returned an unparseable version string"
5. restart vscode
6. use gnuplot --version
7. works
8. use cargo bench -- --plotting-backend gnuplot
9. works

`gnuplot --version` returned an unparseable version string: gnuplot 6.1 patchlevel 0

The version string is in the error message, so it has to be in stdout because parse_version() did get a correct-seeming string. If I copy-paste this same string from the error message and smash it into parse_version(), everything works fine. This suggests the problem is something we cannot see.

Oh hey, that is not UTF-8 that gnuplot is emitting, it is UTF-16! Feeding that into String::from_utf8() is not going to result in happy times.

Why does it show up fine in the error message then? I suppose something somewhere along the pipeline to the error getting printed just skips the 0x00 bytes when displaying.

If all that Criterion needs here is detect version, perhaps a "try again with UTF-16 just in case" fallback might be useful?