The smooth cumulative option makes the data monotonic in x; points with the
same x-value are replaced by a single point containing the cumulative sum of
y-values of all data points with lower x-values (i.e. to the left of the
current data point). This can be used to obtain a cumulative distribution
function from data.
See also
http://www.gnuplot.info/demo/smooth.htmlsmooth.dem
