; This batch file creates a plot used in Chapter 10, "Plotting",
; of _Using IDL_.

;  Create a data array.

X = FLTARR(256)	

; Make a step function. Array elements 80 through 120 are set to 1.

X(80:120) = 1	

; Make a filter.

FREQ = FINDGEN(256)

; Make the filter symmetrical about the value x =128.

FREQ = FREQ < (256-FREQ)

; The filter is a second order Butterworth filter,
; with a cutoff frequency = 20.

FIL = 1./(1+(FREQ/20)^2)

; Plot with a logarithmic x-axis. Use exact axis range.

PLOT, /YLOG, FREQ, ABS(FFT(X,1)), $
    XTITLE = 'Relative Frequency', YTITLE = 'Power', XSTYLE = 1	

; Plot graph. 

OPLOT, FREQ, FIL