plot.c

#define _XOPEN_SOURCE
#include <stdio.h>

int main(void)
{
    FILE *plot = popen("gnuplot", "w");
    fprintf(plot, "plot '-' using 1:2 with lines\n");
    for (int x = -8; x <= 8; x++)
        fprintf(plot, "%d %d\n", x, x * x);
    fprintf(plot, "e\n");
    fflush(plot);
    getchar(); // wait
    return 0;
}