Display a correlogram
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by Sean Becketti, Federal Reserve Bank of Kansas City

          ^ac^ varname [^if^ exp] [^in^ range] [^, nlags(^#^)^]

A graph of the first ^nlag^ autocorrelations is displayed along with a
one-standard-error band.  By default, 20 autocorrelations are plotted.  
The standard error of the autocorrelations is estimated by Bartlett's
approximation.