Sample the original data with replacement and fit LOWESS curves accordingly.
boot.lowess(x, y = NULL, f = 2/3, iter = 3, line.col = "#FF000033", ...)passed to lowess
the color of the LOWESS lines
other arguments passed to the scatterplot by plot
We keep on resampling the data and finally we will see several bootstrapped LOWESS curves, which may give us a rough idea about a ``confidence interval'' of the LOWESS fit.
Examples at https://yihui.org/animation/example/boot-lowess/