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21.1 The histogram struct
A histogram is defined by the following struct,
 Data Type: gsl_histogram

size_t n
This is the number of histogram bins

double * range
The ranges of the bins are stored in an array of n+1 elements pointed to by range.

double * bin
The counts for each bin are stored in an array of n elements pointed to by bin. The bins are floatingpoint numbers, so you can increment them by noninteger values if necessary.

The range for bin[i] is given by range[i] to range[i+1]. For n bins there are n+1 entries in the array range. 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, Here is a diagram of the correspondence between ranges and bins on the numberline for x,
[ bin[0] )[ bin[1] )[ bin[2] )[ bin[3] )[ bin[4] )  x r[0] r[1] r[2] r[3] r[4] r[5] 
In this picture the values of the range array are denoted by r. On the lefthand side of each bin the square bracket ‘[’ denotes an inclusive lower bound (r <= x), and the round parentheses ‘)’ on the righthand side denote an exclusive upper bound (x < r). Thus any samples which fall on the upper end of the histogram are excluded. If you want to include this value for the last bin you will need to add an extra bin to your histogram.
The gsl_histogram
struct and its associated functions are defined
in the header file ‘gsl_histogram.h’.