The smooth frequency option makes the data monotonic in x; points with the
same x-value are replaced by a single point having the summed y-values.
To plot a histogram of the number of data values in equal size bins,
set the y-value to 1.0 so that the sum is a count of occurrences in that bin.
This is done implicitly if only a single column is provided.
Example:
binwidth = <something> # set width of x values in each bin
bin(val) = binwidth * floor(val/binwidth)
plot "datafile" using (bin(column(1))):(1.0) smooth frequency
plot "datafile" using (bin(column(1))) smooth frequency # same result
See also
http://www.gnuplot.info/demo/smooth.htmlsmooth.dem