Resampling from 2D histograms
As in the onedimensional case, a twodimensional histogram made by
counting events can be regarded as a measurement of a probability
distribution. Allowing for statistical error, the height of each bin
represents the probability of an event where (x,y) falls in
the range of that bin. For a twodimensional histogram the probability
distribution takes the form p(x,y) dx dy where,
In this equation
n_{ij} is the number of events in the bin which
contains (x,y),
A_{ij} is the area of the bin and N is
the total number of events. The distribution of events within each bin
is assumed to be uniform.
gsl_histogram2d_pdf

Data Type 
size_t nx, ny
 This is the number of histogram bins used to approximate the probability
distribution function 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 pointed to by yrange.
double * sum
 The cumulative probability for the bins is stored in an array of
nx*ny elements pointed to by sum.

The following functions allow you to create a gsl_histogram2d_pdf
struct which represents a two dimensional probability distribution and
generate random samples from it.
gsl_histogram2d_pdf * gsl_histogram2d_pdf_alloc (size_t nx, size_t ny)

Function 
This function allocates memory for a twodimensional probability
distribution of size nxbyny and returns a pointer to a
newly initialized gsl_histogram2d_pdf struct. If insufficient
memory is available a null pointer is returned and the error handler is
invoked with an error code of GSL_ENOMEM .

int gsl_histogram2d_pdf_init (gsl_histogram2d_pdf * p, const gsl_histogram2d * h)

Function 
This function initializes the twodimensional probability distribution
calculated p from the histogram h. If any of the bins of
h are negative then the error handler is invoked with an error
code of GSL_EDOM because a probability distribution cannot
contain negative values.

void gsl_histogram2d_pdf_free (gsl_histogram2d_pdf * p)

Function 
This function frees the twodimensional probability distribution
function p and all of the memory associated with it.

int gsl_histogram2d_pdf_sample (const gsl_histogram2d_pdf * p, double r1, double r2, double * x, double * y)

Function 
This function uses two uniform random numbers between zero and one,
r1 and r2, to compute a single random sample from the
twodimensional probability distribution p.
