Project: svg-chart
Task (moderate): 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