; docformat = 'rst'
;+
;This function takes the linear interpolation along line (x1, y1) to 
;(x2, y2) at point x.
;
; :Examples:
;   result = linterp(x1, y1, x2, y2, x)
;-
;+
; :Params:
;   x1 : in, required, type=double
;     X-coordinate of the first point on the line. Must be a scalar.
;   y1 : in, required, type=double or array of double
;     Y-coordinate(s) of the first point on the line. May be a scalar or 
;     an array as long as it agrees dimensionally with y2.
;   x2 : in, required, type = double
;     X-coordinate of the second point on the line. Must be a scalar.
;   y2 : in, required, type=double or array of double
;     Y-coordinate(s) of the second point on the line. May be a scalar or
;     an array as long as it agrees dimensionally with y1.
;   x : in, required, type=double or array of double
;     X-coordinate of where the linear interpolation is calculated. May be
;     an array, if y1 and y2 are scalar.
;     
; :Returns: 
;   Returns the linear interpolation between the points (x1, y1) and 
;   (x2, y2).
;-
function linterp,x1,y1,x2,y2,x
  
  if (n_elements(x1) ne 1) or (n_elements(x2) ne 1) then begin
    doc_library, 'linterp'
    return, -1
  endif
  t=(x-x1)/(x2-x1)
  return,y1*(1-t)+y2*t
end