To give an example, if you have a stacked column chart with two columns, but only one column has data for both series, it would be good to be able to round the corners (currently using the rounded-corners.js plugin) of whichever data point is on top of the stacked column. Currently, borderWidth and borderColor can be specified for individual points. https://api.highcharts.com/highcharts/series.column.data.borderColor
As you can see in the documentation, there is no borderRadius option.