Unexpected behaviour with negative fraction of seconds in xdata time

Question

Can anybody explain the strange behaviour of gnuplot with negative fractions of seconds when plotting with xdata time? I mean the zig-zag in the middle plot. Is this a bug or am I missing anything? (gnuplot 5.2.6)

Code:

### strange behaviour with negative fractions of seconds in xdata time
reset session

set table $Data
    plot '+' u 1:($1**2) w table
unset table

set multiplot layout 3,1
    plot $Data u 1:2 w lp pt 7 lc rgb "web-green"

    set xdata time
    set timefmt "%s"
    set format x "%H:%M:%S"
    plot $Data u 1:2 w lp pt 7 lc rgb "red"

    set format x "%H:%M"
    plot $Data u ($1*60):2 w lp pt 7 lc rgb "web-blue"

unset multiplot
### end of code

Result:

Answer 1

You are over-complicating this. Your data is purely in seconds. No need to invoke "set xdata time" or "set timefmt" variants because the input is just a floating point number. On output, set the xtic format depending on whether you want the x axis labeled in absolute time (wraps at midnight) or relative time (+/- interval).

### absolute vs relative time formats
set table $Data
    plot '+' u 1:($1**2) w table
unset table

set multiplot layout 3,1
    plot $Data u 1:2 w lp pt 7 lc rgb "web-green"

    set xtics time format "%tH:%tM:%tS" # relative time
    plot $Data u 1:2 w lp pt 7 lc rgb "red"

    set xtics time format "%H:%M:%S"    # absolute time
    plot $Data u 1:2 w lp pt 7 lc rgb "blue"
unset multiplot
### end of code

But no, sorry, I cannot explain why your original plot came out with a zigzag.