!  Sunspot example

DEFINE AXIS/T="1-jul-1700":"1-jul-1997":1/unit=yr tsun
DEFINE GRID/T=tsun gsun
FILE/VAR=sunspot_index/skip=3/G=gsun/FORMAT=(6x,f6.1) yearssn.dat

!SET MODE META
PLOT/SET_UP sunspot_index
PPLUS TXLSZE 0.07
PPL PLOT
MESSAGE

LET nd = sunspot_index[l=@NGD]
LET nf = nd / 2
LET yquist = 0.5
LET freq1 = yquist/ nf

DEFINE AXIS/t=`freq1`:`yquist`:`freq1`/UNITS=yr FAXIS
DEFINE GRID/t=faxis gfreq
SET GRID gfreq
LET tpts = t

LET fftsun = ffta(sunspot_index[l=1:`nd`])

PLOT fftsun
message

!  Show the same plot in years/cycle
LET fpts = 1./tpts

let nff = fpts[l=@NGD]
let nfs = fftsun[l=@NGD]
let nfplot = MIN(`nff`, `nfs`)

PLOT/VS/LINE/SET_UP fpts[l=1:`nfplot`], fftsun[l=1:`nfplot`]
PPL XLAB YR/cycle
PPL YLAB Amplitude Spectrum
PPL XAXIS 0, 30, 1.
PPL PLOT
message

!  Plot the power spectrum in years/cyc

LET pspect = fftsun*fftsun
PLOT/VS/LINE/SET_UP fpts[l=1:`nfplot`], pspect[l=1:`nfplot`]
PPL XAXIS 1, 30, 1
PPL XLAB years/cycle
PPL YLAB Power Spectrum
PPL PLOT
MESSAGE