[Feature request] Stacked Bar Radius

[CalebKester]

Description

The radius setting for bar charts read NOTE: Works only for non-stacked bar. I'm wondering if there is a way to add in rounded for data groups?

Steps to check or reproduce

add the following groups to this graph https://naver.github.io/billboard.js/demo/#BarChartOptions.BarRadius

// import bb, {bar} from "billboard.js"

var chart = bb.generate({
  data: {
    columns: [
	["data1", 100, 250, -200, 200, 250, 150],
	["data2", 170, -350, 240, 200, -250, 150],
	["data3", -120, 100, 240, -300, 350, 350],
	["data4", 180, 130, 340, 200, 250, -250]
    ],
    groups: [["data1", "data2"]],
    type: "bar", // for ESM specify as: bar()
  },
  bar: {
    radius: {
      ratio: 0.5
    }
  },
  bindto: "#barRadius"
});