[Gnuplot-info] columns with time format as "y"

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Hello All,

I need help with the following problem:

Let's say my input data look like (columns delimited by \t):

   2019-04-16 20:30:00	30522	5
   2019-04-16 20:31:00	30523	6
   2019-04-16 20:32:00	30524	7
   2019-04-16 20:33:00	30525	8
   2019-04-16 20:34:00	30526	9
   2019-04-16 20:35:00	30527	10
   2019-04-16 20:36:00	30528	11
   2019-04-16 20:37:00	30529	12
   2019-04-16 20:38:00	30530	13
   2019-04-16 20:39:00	30531	14

Column 1 & 2 (space separated) considered as date/time.

Plotting Col. 3 or 4 against date/time works fine:

   set xdata time
   set timefmt "%Y-%m-%d\t%H:%M:%S"
   set format x "%d.%m. %H:%M"
   plot 'my_file.txt' using 1:3 title "col 3" axis x1y1 with linespoints
     pt7 lc 4, '' using 1:4 title "col 4" axis x1y1 with linespoints
     pt7 lc 7

Special feature: Although columns 1 and 2 *together* give the timestamp, 
only "1:..." is given for "using" statement. This method is often found 
in examples.

But now I would like - don't ask me why - to plot the timestamp against 
column 3, i.e. column 3 as "x" and the timestamp (columns 1 & 2) as "y".

I was trying:

   set ydata time
   set timefmt "%Y-%m-%d\t%H:%M:%S"
   set format y "%d.%m. %H:%M"
   plot 'my_file.txt' using 3:1 title "col 3" axis x1y1 with
     linespoints pt7 lc 4

Result: only date is used for plotting "y", time ignored!

Another trial:

   plot 'my_file.txt' using 3:($1+$2) title "col 3" axis x1y1 with
     linespoints pt7 lc 4

That didn't work either: this time the time was used for the plot as 
"y", but the date was set to 1.1. and the true date from column 1 was 
ignored.

What am I doing wrong? Unfortunately, you won't find an example of a 
timestamp consisting of two columns as a "y" value anywhere. Admittedly, 
this is rarely of use.

Thanks!

Best,
Thomas