GSL::Histogram2d class

# Title: GSL::Histogram2d class

## 1 Singleton Methods

`GSL::Histogram2d.new(nx, ny)`
`GSL::Histogram2d.new(nx, xmin, xmax, ny, ymin, ymax)`
  Constructors
`GSL::Histogram2d.new_uniform(nx, xmin, xmax, ny, ymin, ymax)`
`GSL::Histogram2d.alloc_uniform(nx, xmin, xmax, ny, ymin, ymax)`
`GSL::Histogram2d.equal_bins_p(h1, h2)`

## 2 Methods

`GSL::Histogram2d#set_ranges(vx, vy)`
  This sets the ranges of the existing histogram using GSL::Vector objects.
`GSL::Histogram2d#set_ranges_uniform(xmin, xmax, ymin, ymax)`
`GSL::Histogram2d#clone`
`GSL::Histogram2d#increment(x, y)`
`GSL::Histogram2d#accumulate(x, y, weight)`
`GSL::Histogram2d#get(i, h)`
`GSL::Histogram2d#get_xrange(i)`
`GSL::Histogram2d#get_yrange(j)`
`GSL::Histogram2d#xmax`
`GSL::Histogram2d#xmin`
`GSL::Histogram2d#ymax`
`GSL::Histogram2d#ymin`
`GSL::Histogram2d#nx`
`GSL::Histogram2d#ny`
`GSL::Histogram2d#reset`
`GSL::Histogram2d#find(x, y)`
`GSL::Histogram2d#max_val`
`GSL::Histogram2d#max_bin`
`GSL::Histogram2d#min_val`
`GSL::Histogram2d#min_bin`
`GSL::Histogram2d#xmean`
`GSL::Histogram2d#xsigma`
`GSL::Histogram2d#ymean`
`GSL::Histogram2d#ysigma`
`GSL::Histogram2d#cov`
`GSL::Histogram2d#sum`
`GSL::Histogram2d#add`
`GSL::Histogram2d#sub`
`GSL::Histogram2d#mul`
`GSL::Histogram2d#div`
`GSL::Histogram2d#scale`
`GSL::Histogram2d#shift`
`GSL::Histogram2d#fwrite(io)`
`GSL::Histogram2d#fwrite(filename)`
`GSL::Histogram2d#fread(io)`
`GSL::Histogram2d#fread(filename)`
`GSL::Histogram2d#fprintf(io, range_format, bin_format)`
`GSL::Histogram2d#fprintf(filename, range_format, bin_format)`
`GSL::Histogram2d#fscanf(io)`
`GSL::Histogram2d#fscanf(filename)`

## 3 GSL::Histogram2d_pdf class

`GSL::Histogram2d_pdf.new(nx, ny)`
`GSL::Histogram2d_pdf.alloc(nx, ny)`
  Constructors
`GSL::Histogram2d_pdf#init(h)`
  Initializer with a GSL::Histogram2d object h|
`GSL::Histogram2d_pdf#sample(r1, r2)`

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