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21.13 The 2D histogram struct
Two dimensional histograms are defined by the following struct,
 Data Type: gsl_histogram2d

size_t nx, ny
This is the number of histogram bins in the x and y directions.

double * xrange
The ranges of the bins in the xdirection are stored in an array of nx + 1 elements pointed to by xrange.

double * yrange
The ranges of the bins in the ydirection are stored in an array of ny + 1 elements pointed to by yrange.

double * bin
The counts for each bin are stored in an array pointed to by bin. The bins are floatingpoint numbers, so you can increment them by noninteger values if necessary. The array bin stores the two dimensional array of bins in a single block of memory according to the mapping
bin(i,j)
=bin[i * ny + j]
.

The range for bin(i,j)
is given by xrange[i]
to
xrange[i+1]
in the xdirection and yrange[j]
to
yrange[j+1]
in the ydirection. Each bin is inclusive at the lower
end and exclusive at the upper end. Mathematically this means that the
bins are defined by the following inequality,
Note that any samples which fall on the upper sides of the histogram are
excluded. If you want to include these values for the side bins you will
need to add an extra row or column to your histogram.
The gsl_histogram2d
struct and its associated functions are
defined in the header file ‘gsl_histogram2d.h’.