Please show or attach the PostScript code that is produced.
Otherwise it is impossible to say whether the problem is in gnuplot or in your installed fonts or in your PostScript viewer program. I'm guessing it's the viewer, becuase if it were a font problem I would expect that you would see a "q" in the output.
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Has anyone else had issues with symbols not appearing in the potscript terminal? For example:
set label "{/Symbol = q}"
will not show anything on the plot. I'm on macOS Sierra.
Please show or attach the PostScript code that is produced.
Otherwise it is impossible to say whether the problem is in gnuplot or in your installed fonts or in your PostScript viewer program. I'm guessing it's the viewer, becuase if it were a font problem I would expect that you would see a "q" in the output.