A numeric giving the increment to adjust. Sometimes low values
are needed to find a proper adjust value.
Details
This function finds the first value of the stats::density()adjust
argument which gives the .k number of clusters. It it quite crude in that
every value of adjust from .lower to .upper is tested until the desired
number of clusters is found. A cluster is defined by a peak, and should no
suitable adjust value be found, NA is returned.