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From: John Edwards <johned0@gmail.com>
Newsgroups: comp.graphics.apps.gnuplot
Subject: problem piping commands to gnuplot under Windows
Date: Sun, 12 Jan 2014 00:19:16 +0000
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Hello again folks,

I am trying to plot two data files using the following code :

#include <stdlib.h>
#include <stdio.h>

void main()
{
   FILE * gnuplotPipe = _popen ("gnuplot -persist", "w");
   FILE * f1 = fopen("1.tmp", "w");
   FILE * f2 = fopen("2.tmp", "w");
   int i;

   for (i = 0; i < 5; i++)
   {
     fprintf (f1, "%lf %lf\n", (float)i, (float)i);
     fprintf (f2, "%lf %lf\n", (float)i, (float)i+1);
   }
   fclose (f1); fclose (f2);

   fprintf (gnuplotPipe, "plot \"1.tmp\" using 1:2 title \"Data set 0\" 
with lines lc rgb \"purple\", \\\n");
   fprintf (gnuplotPipe, " \"2.tmp\" using 1:2 title \"Data set 1\" with 
lines lc rgb \"green\"\n");
   fflush (gnuplotPipe);
   getchar ();
   fprintf (gnuplotPipe, "exit\n");
   _pclose (gnuplotPipe);
}

This works fine if I run the plot command in gnuplot from a script or if 
I use pgnuplot but when I use gnuplot I get the following error message :

gnuplot> plot "1.tmp" using 1:2 title "Data set 0" with lines lc rgb 
"purple", \
 >
gnuplot>  '2.tmp' using 1:2 title "Data set 1" with lines lc rgb "green"
          ^
          invalid command

Can anyone tell me if I am being a numpty ?

Thanks very much,
John