;+
; NAME:
;    xy2psym10
; PURPOSE:
;    Simulate the !p.psym=10 (histogram type plotting) setting but
;    return the actual data values used to do this. Thus filled
;    histograms can be made using polyfill.
;
; CALLING SEQUENCE:
;    XY2PSYM10,X,Y
;
; INPUTS:
;    X,Y = The original data to be plotted
;
; OUTPUTS:
;    X,Y = Output data is nearly 2 X size of the input x,y contains
;    the vertices defining a histogram style plot
;
; EXAMPLE:
;    Given x and y, 
;      IDL> !p.psym = 0
;      IDL> xy2psym10,x,y
;      IDL> plot,x,y
;      IDL> polyfill,x,y,color=150
;
; MODIFICATION HISTORY:
;    Written by: Trevor Harris, Physics Dept., University of Adelaide,
;    July, 1990.
;-
pro xy2psym10,xx,yy

  num = n_elements(xx)
  inc = (xx(1)-xx(0))*0.5
  maxx = max(xx,min=minx)
  x = fltarr(2*num)
  y = x
  i = indgen(num)*2.
  x(i) = xx-inc
  y(i) = yy
  x(i+1) = xx+inc
  y(i+1) = yy
  x = ([minx,x,maxx] < maxx) > minx
  y = [0,y,0]

  xx = x
  yy = y

  return
end