Thanks!
No I don’t know y values in advance. They are determined by counting the number of occurences of each value per bin.
When I pipe the stdout of pshistogram to psxy I still get a clean sheet. I’m afraid, psxy requires the same -J or -T or -R options which I don’t understand how to use in the first place.

It was a combination of things that I did wrong.
When I added -B options for axes labels, I got my first non-blank sheet which was very helpful to identify my mistakes:
There was a syntax error in my -J option (no “y” behind the separating slash).
After I fixed that, I got more output to play with and started to understand the effects of my changes.

Here’s what I came up with eventually: gmt pshistogram <timestamp.log>-JX25T/10 -TT00:00:00/T23:59:59/1440+n -fT -Bx2H+l"Time" -By+l"Frequency" -BWS -Gblack

My options explained as I understood them (for anybody interested): -JX25T/10 chart dimensions in absolute values, 25cm wide and time based, 10cm high -TT00:00:00/T23:59:59/1440+n histogram bins from min value 00:00 to max value 23:59:59 with 1440 bins (= minutes per day) -fT use abolute time based -Bx2H+l"Time" x-axis labeled “Time” with 2 hour ticks -By+l"Frequency" y-axis labeld “Frequency” with auto-/default-ticks -BWS show only left (west) and bottom (south) axes, hide top and right -Gblack fill bars black