Project: chart
Task (challenging): In `./common/PieChart.js`, when `options.dataLabels` is true:
- append g element (class `dataLabels dataLabels-0`) inside `svg` for the first `dataset`
- append data labels (each class `dataLabel dataLabel-{index}`) inside `.dataLabels` for each `dataset.data` item
- show the sector data and proportion in each dataLabel
- append connect lines (each class `connect connect-{index}`) inside `.dataLabels` to connect each dataLabel and its corresponding sector
- each dataLabel is positioned near its corresponding sector and distributed around the large circle