Project: svg-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

Update `index.js`, create `AreaChart` instances when `config.type` is `area`.