* Draw species-area curve
program define esarea
version 2.1
    ***         CLEAR DUMMY VARIABLES
capture drop V1
capture drop V2
capture drop V3
    ***         DEFINE VARIABLES & STANDARD AXIS LABELS
macro define _varlist "required existing "
macro define _options "*"
parse "%_*"
parse "%_varlist", parse(" ")
macro define a "l1(Cumulative Number of Species)"
macro define b "b2(Number of Samples/ Relative Area Sampled)"
macro define c "b1(Species-Area Curve)"
    ***         ENTER AXIS MAXIMA
   generate V2=sum(%_1)
di " "
di "To control scale of the X-axis, enter a maximum value (must be>" _N
di "  OR  Press <Enter> to let STATA select X-axis scales."
di "Enter X-axis maximum (or press <Enter>)..." _r(xmax)
di " "
di "To control scale of the Y-axis, enter a maximum value (>" V2[_N] ")"
di "  OR  Press <Enter> to let STATA select Y-axis scales."
di "Enter Y-axis maximum (or press <Enter>)..." _r(ymax)
    ***           USE DEFAULT MAXIMA, if necessary
if "%xmax"=="" {macro define xmax=_N  }
if "%ymax"=="" {macro define ymax =V2[_N]  }
confirm number %xmax
confirm number %ymax
    ***           GET GRAPH TITLE
di " "
di "Enter a title to label this graph,"
di "    OR press <Enter> to use " in red "   %_1  " in white " as a label."
di "Title..." _r(label)
di " "
if "%label"=="" {macro define label "%_1"}
macro define label "t2(%label)"
    ***           ANCHOR X-AXIS AT 0, by adding a null observation.
macro define d=_N+1
quietly {
      set obs %d
      generate V1=_n-1
      generate V3=V2[_n-1]
      replace V3=0 if _n==1 }
   ***           GRAPH WITH INTERACTIVES title (%label), xmax (%xmax) and ymax (%ymax)
gr V3 V1, c(l) xla yla xscale(0,%xmax) yscale(0,%ymax) %a %b %c %label %_options
   drop V1 V2 V3
   capture drop if _n==1
end