[Gnuplot-info] What does 'line 0' mean?

[Aleksey T. <ats...@gm...>]

As a Web application owner, I wanted to graph the number of daily unique
visitors (out of my Apache httpd log) for the last six weeks.

My httpd log goes 5 months back (from July to November 2017) -- here it is
sorted by month:

    # cat /var/cfengine/httpd/logs/access_log* |   awk '/GET \/api\/user\//
{print $4, $7}'|   cut -b2-12,22- |   grep -v admin  |   sort -u |   sort
 -k2  -t/ -M |   awk '{print $1}' |   uniq -c |   awk 'NR == 1 { print }
END { print }'
          1 12/Jul/2017
          2 09/Nov/2017
    #



I tried the following:

    set xdata time
    set xtics rotate
    set timefmt "%d/%b/%Y"
    set format x "%d/%b/%Y"

    set autoscale xfixmin
    set autoscale xfix

    # only show visitors for last 6 weeks (sliding window)
    min=system("date -d '6 weeks ago' +%d/%h/%Y")
    max=system("date -d 'now' +%d/%h/%Y")
    set xrange [ "min" : "max"]
    show xrange

    plot '-' using 2:1 with lines


which generated:

             line 0: illegal day of month


            set xdata time
            set xrange [ * : * ] noreverse nowriteback  # (currently
["01/Jan/2000":"01/Jan/2000"] )
    set autoscale xfixmin
    set autoscale xfixmax

So then I took out the xrange stuff and instead just added "| tail -42" to
my one-liner:

    cat /var/cfengine/httpd/logs/access_log* |   awk '/GET \/api\/user\//
{print $4, $7}'|   cut -b2-12,22- |   grep -v admin  |   sort -u |   sort
 -k2  -t/ -M |   awk '{print $1}' |   uniq -c |  tail -42

I can feed THAT into gnuplot and it does what I want, graphs the last six
months only. Yay! So I've solved my graphing needs, but I'd like to
understand the error message.  What does 'line 0' mean?  There is nothing
wrong with my input...  What am I doing wrong with the gnuplot commands?

Love the tool, by the way. So powerful and useful!

Thanks!
Aleksey