Making C code plot a graph automatically

Question

I have written a program which writes a list of data to a '.dat' file with the intention of then plotting it separately using gnuplot. Is there a way of making my code plot it automatically? My output is of the form:

x-coord    analytic    approximation
x-coord    analytic    approximation
x-coord    analytic    approximation
x-coord    analytic    approximation
x-coord    analytic    approximation
 ....

Ideally, when I run the code the graph would also be printed with an x-label, y-label and title (which could be changed from my C code). Many thanks.

Answer 1

I came across this while searching for something else regarding gnuplot. Even though it's an old question, I thought I'd contribute some sample code. I use this for a program of mine, and I think it does a pretty tidy job. AFAIK, this PIPEing only works on Unix systems (see the edit below for Windows users). My gnuplot installation is the default install from the Ubuntu repository.

#include <stdlib.h>
#include <stdio.h>
#define NUM_POINTS 5
#define NUM_COMMANDS 2

int main()
{
    char * commandsForGnuplot[] = {"set title \"TITLEEEEE\"", "plot 'data.temp'"};
    double xvals[NUM_POINTS] = {1.0, 2.0, 3.0, 4.0, 5.0};
    double yvals[NUM_POINTS] = {5.0 ,3.0, 1.0, 3.0, 5.0};
    FILE * temp = fopen("data.temp", "w");
    /*Opens an interface that one can use to send commands as if they were typing into the
     *     gnuplot command line.  "The -persistent" keeps the plot open even after your
     *     C program terminates.
     */
    FILE * gnuplotPipe = popen ("gnuplot -persistent", "w");
    int i;
    for (i=0; i < NUM_POINTS; i++)
    {
    fprintf(temp, "%lf %lf \n", xvals[i], yvals[i]); //Write the data to a temporary file
    }

    for (i=0; i < NUM_COMMANDS; i++)
    {
    fprintf(gnuplotPipe, "%s \n", commandsForGnuplot[i]); //Send commands to gnuplot one by one.
    }
    return 0;
}

EDIT

In my application, I also ran into the problem that the plot doesn't appear until the calling program is closed. To get around this, add a fflush(gnuplotPipe) after you've used fprintf to send it your final command.

I've also seen that Windows users may use _popen in place of popen -- however I can't confirm this as I don't have Windows installed.

EDIT 2

One can avoid having to write to a file by sending gnuplot the plot '-' command followed by data points followed by the letter "e".

e.g.

fprintf(gnuplotPipe, "plot '-' \n");
int i;

for (int i = 0; i < NUM_POINTS; i++)
{
  fprintf(gnuplotPipe, "%lf %lf\n", xvals[i], yvals[i]);
}

fprintf(gnuplotPipe, "e");

Answer 2

You can either create a gnuplot script and spawn a process running gnuplot to plot this script from the commandline, or you may use one of the provided interfaces. For C, there is a POSIX pipe-based interface from Nicolas Devillard available here: http://ndevilla.free.fr/gnuplot/ ...and an iostream-based C++ version is available via git (see: http://www.stahlke.org/dan/gnuplot-iostream/ )

The most portable and probably the easiest way would still be calling gnuplot to plot a script, though. As sje397 mentioned, check your documentation for the system() call in stdlib.h.

On a sidenote, there is also GNU plotutils, which offers libplot, a library for plotting datasets, which you could use in your application. See: http://www.gnu.org/software/plotutils/

Answer 3

Although I've seen a lot of ways of doing this, the most simplest way of doing this would be by using the system() (from stdlib.h) function in C. First make a gnuplot script and save it as "name.gp" (neither the name nor the extension matter).
A simple script would be,

plot 'Output.dat' with lines

After saving this script file, just add
system("gnuplot -p 'name.gp'");
at the end of your code.
It's as simple as that.

Answer 4

I've adapted the accepted answer to plot a float array while avoiding the use of a temporary file. In it, float* data_ is the array and size_t size_ its size. Hopefully it is helpful for someone!

Cheers,
Andres

void plot(const char* name="FloatSignal"){
  // open persistent gnuplot window
  FILE* gnuplot_pipe = popen ("gnuplot -persistent", "w");
  // basic settings
  fprintf(gnuplot_pipe, "set title '%s'\n", name);
  // fill it with data
  fprintf(gnuplot_pipe, "plot '-'\n");
  for(size_t i=0; i<size_; ++i){
    fprintf(gnuplot_pipe, "%zu %f\n", i, data_[i]);
  }
  fprintf(gnuplot_pipe, "e\n");
  // refresh can probably be omitted
  fprintf(gnuplot_pipe, "refresh\n");
}