#2255 cannot read png/gif/jpeg images

[Ryo]

On a Linux machine ( I don't know what distribution it is. uname -a says, 2.6.32-754.22.1.el6.x86_64 ), I downloaded gnuplot-5.2.8.tar.gz, configured it without any options other than --prefix and launched gnuplot. It seems to be working, but

gnuplot> plot "sample.png" binary filetype=png
         This copy of gnuplot cannot read png/gif/jpeg images
 gnuplot> show datafile binary filetypes

    This version of gnuplot understands the following binary file types:
      avs     bin     edf     ehf     gif     gpbin   jpeg    jpg     png     raw     rgb     auto

Perhaps this is just a matter of the wording of these messages and png isn't really supported?

[Ethan Merritt]

I can see how that is a bit confusing.
The command show data binary filetypes lists the filetypes that gnuplot is ready to attempt to open. The error message isn't triggered until you actually attempt to open one. So essentially it's telling you "I know how to read these but this copy of the program wasn't linked to the library of routines I would need".

The library in question is libgd. When you ran the ./configure script it should have printed out a summary of all the libraries it could or could not find. There should be a line like this:
libgd-based png, jpeg, and gif terminals: no (requires libgd, see config.log).

Your machine is apparently running RHEL 6. There is probably a libgd package available for it, or you could use the one from CentOS: https://centos.pkgs.org/6/centos-x86_64/gd-devel-2.0.35-11.el6.x86_64.rpm.html

Install libgd, re-run configure + install for gnuplot, and you should be all set.

[Ryo]

I see! Thank you for your clear and detailed explanation and sorry for my false report. At least, because this issue tracker is searchable (correct?), I hope some people may find this question & answer useful in the future.