Project: chart
Task (challenging): - In `./common/AreaChart.js`, class `AreaChart` inherits from `LineChart`, `AreaChart`:
  - append g element (class `areas`) inside `svg`
  - append polygons (each class `area area-{index}`) inside `.areas` 
  - each `.area` is below the `.dataset` line
  - each `.area` color is the same as `.dataset` line color, half transparent
- Select the option `Area Chart` of `#type` to create `AreaChart` instance