about error message "matrix does not represent a grid" in gnuplot

[Dawn]

Hi, all:

I used fopen in c to read in a data file. I used plot 'data.txt' matrix with image to plot out the image. However, when I used fopen to write in the same data set as a new data file and I used same command line with new data file as: plot 'data1.txt' matrix with image, the image cannot be plotted and error message said:

matrix does not represent a grid


My basic codes are:


------------------------------------------------------


int main()
{
#ifdef WIN32
FILE *pipe = _popen(GNUPLOT_NAME, "w");
#else
FILE *pipe = popen(GNUPLOT_NAME, "w");
#endif

int xsize = 20, ysize = 41;
float **din;

// alloc memory
din = (float **)malloc(sizeof(float *) * ysize);


for (int i = 0; i < ysize; i++) {
din[i] = (float *)malloc(sizeof(float) * xsize);
}


// read in 2D data
FILE *fp, *fp1;
fp = fopen("wedgegas.txt", "r");
fp1 = fopen("inversedwedgegas.txt", "w");


// read in data
if (fp == NULL) {
printf("fp is NULL!\n");
exit(1);
}

for (int i = 0; i < ysize; i++) {
printf("i=%d \n ", i);
for (int j = 0; j < xsize; j++) {
fscanf(fp, "%f ", &din[i][j]);
printf("%f ", din[i][j]);
}
fscanf(fp, "\n");
printf("\n ");
}


/* write final inversed data to din */
if (fp1 == NULL) {
printf("fp1 is NULL!\n");
exit(1);
}
for (int i = 0; i < ysize; i++) {
printf("%d \n", i);
for (int j = 0; j < xsize; j++) {

fprintf(fp1, "%f ", din[i][j]);
printf("%f ", din[i][j]);
}
fprintf(fp1, "\n");
printf("\n");
}


/* ---------------------------- plot results ------------------------------------------------------- */
plot:
if (pipe != NULL)
{
// plot sinc wave plot 1
fprintf(pipe, "set multiplot layout 1,2 \n"); // set multiple plots


fprintf(pipe, "plot 'wedgegas.txt' matrix with image \n"); // plot type



fprintf(pipe, "%s \n", "e"); // termination character, end a plot
fflush(pipe); // flush the pipe



fprintf(pipe, "plot 'inversedwedgegas.txt' matrix with image \n"); // plot type

fprintf(pipe, "%s\n", "e"); // termination character
fflush(pipe); // flush the pipe


// wait for key press
std::cin.clear();
std::cin.ignore(std::cin.rdbuf()->in_avail());
std::cin.get();

#ifdef WIN32
_pclose(pipe);
#else
pclose(pipe);
#endif
}
else
std::cout << "Could not open pipe" << std::endl;



fclose(fp);
fclose(fp1);



return 0;
}

------------------------------------------------------------

Any suggestions? Thanks in advance!