Project: chart
Task (challenging): - In `./common/PieChart.js`, class `PieChart` inherits from `Chart`, `PieChart`:
  - append g element (class `datasets`) inside `svg` and make it fill the most area of `svg`
  - append g element (class `dataset dataset-0`) inside `.datasets` for the first `datasets` item
  - append sectors (path elements, each class `sector sector-{index}`) with the unique color inside `.dataset` for each `dataset.data` item
  - all the sectors form a large circle
  - the area of each sector is related to its proportion of the data
- Select the option `Pie Chart` of `#type` to create `PieChart` instance, make `#axes`, `#grids`, `#pointStyle` disabled, and make `#datasets` single selection mode