This function implements a rule of thumb for selecting the bandwidth when
smoothing a function of metric values per cutpoint value, particularly
in maximize_loess_metric
and minimize_loess_metric
.
user_span_cutpointr(data, x)
A data frame
The predictor variable
The function used for calculating the bandwidth is 0.1 * xsd / sqrt(xn), where xsd is the standard deviation of the unique values of the predictor variable (i.e. all cutpoints) and xn is the number of unique predictor values.