Project: chart
Task (moderate): - Check or uncheck `#dataLabels` to create `LineChart` instance:
  - reuse function `createChart()`
  - add new `options` attribute:
    - dataLabels: the `checked` value of `#dataLabels`
- In `./common/LineChart.js`, when `options.pointStyle` is `circle` or `rect`:
  - append g element (class `points points-{index}`) inside `svg` for each `dataset`
  - append points (each class `point point-{index}`, shape is `options.pointStyle`) inside `.points` for each item of `dataset.data`
  - each point covers the `.dataset` line, distributes from left to right
  - each point color is the same as `.dataset` line color