gnuplot.c: enable correct terminal for non MINGW/MSVC

src/frontend/plotting/gnuplot.c

@@ -110,6 +110,9 @@ ft_gnuplot(double *xlims, double *ylims, char *filename, char *title, char *xlab
     }
 
     /* Set up the file header. */
+#if !defined(__MINGW__) && !defined(_MSC_VER)
+    fprintf(file, "set terminal X11\n");
+#endif
     if (title) {
         text = cp_unquote(title);
         fprintf(file, "set title \"%s\"\n", text);