Styjun

I use C++ in Xcode on Mac and a pipe method to communicate with Gnuplot. I am interested in converting my arrays into graphs directly through the program, after I run it. Using

FILE *f = popen("gnuplot -persist", "w");

I open the file and then communicate using fprintf.

Now, I have some data in arrays of interest. w is a "proposed" array of standard normal variables and I intend to check if it is indeed a Gaussian distribution with mean = 0 and variance = 1 .To do that I plot a histogram. After that I want to superimpose a real Gaussian function, which has ex as a x coordinate values and gauss as y coordinate values directly on the histogram. How can I do that?

Here's the code so far:

double start = -4; //min
double end = 4 ; //max
double numberofbins = 100;
double width = (end-start)/numberofbins ;

fprintf (f,
 "set ylabel '# of elements'n"
 "set xlabel 'The numbers'n"
 "Min = %gn" //where binning starts
 "Max = %gn" // where binning ends
 "n = %gn" // the number of bins
 "width = 10**(-1)n" // binwidth; (Max-Min)/n
 "bin(x) = width*(floor((x-Min)/width)+0.5) + Minn"
 "f(x)= e**((-x**2)/2) / sqrt(2*pi)n"
 "plot '-' using (bin($1)):(1) smooth freq with boxes,'' u $2:$3 with lines linestyle 1n",start,end,numberofbins)

for (int i= 0; i < numberofpoints; i++)
 fprintf(f, "%g %g %gn", w[i], ex[i], gauss[i]);


fclose(f);

Here is the result if I run the demonstrated code:

As we can see, the binning was successful but the line was omitted and gives the following error:

gnuplot> plot '-' using (bin($1)):(1) smooth freq with boxes,'' u $2:$3 with lines linestyle 1
 ^
 line 100000: column() called from invalid context

I have checked online but nobody is practicing communicating with Gnuplot that way.

If I plot only the 2:3 part (without binning), I get this graph:

Thus, the problem might be with the compatibility of these two plots.

there are different ways to plot "inline" data

plot '-' u 1:2 w lines
1 11
2 22
3 33
e

From gnuplot help special-filenames

If you use both '-' and '' on the same plot command, you'll need to
have two sets of inline data, ...

This means:

plot '-' u 1:2 w boxes, '' u 1:2 w lines
1 11
2 22
3 33
e
1 11
2 22
3 33
e

So, instead, I would generate a datablock in the beginning of your generated command string and reuse the data as many times as you need it during your plotting command.

$Data <<EOD
1 11
2 22
3 33
EOD

plot $Data u 1:2 w boxes, '' u 1:2 w lines

I have solved the problem by creating a second y axis on the same graph and plotting according to it. The code used was:

 fprintf (f,
 "set xlabel 'The numbers'n"
 "Min = %gn" //where binning starts
 "Max = %gn" // where binning ends
 "n = %gn" // the number of bins
 "width = 10**(-1)n" // binwidth; (Max-Min)/n
 "bin(x) = width*(floor((x-Min)/width)+0.5) + Minn"
 "set ytics 100 nomirror tc lt 1n"
 "set ylabel '# of elements' tc lt 1n"
 "set y2tics 0.4 nomirror tc lt 2n"
 "set y2label 'Theoretical Gaussian' tc lt 2n"
 "plot '-' using (bin($1)):(1) smooth freq with boxes title 'Generator Histogram','-' u 1:2 with l axes x1y2 title 'Theoretical Gaussian (mean=0, std = 1)'n",start,end,numberofbins) ;

 for (int i= 0; i < numberofpoints; i++)
 fprintf(f, "%gn", w[i]);
 
 fprintf(f,"en");

 for (int i= 0; i < numberofpoints; i++)
 fprintf(f, "%g %gn",ex[i], gauss[i]);
 
 fprintf(f,"en");
 fclose(f); 

which plots this: