Project: svg-chart
Task (challenging): In `common/BarChart.js`, class `BarChart` inherits from `Chart`, `BarChart`:
- there is 1 more grid-x line than LineChart, all the grid-x lines are evenly distributed in `.grids`
- append g element (class `datasets`) inside `svg` and make it fill the entire `svg` (no padding between them)
- append g element (class `dataset dataset-${index}`) inside `.datasets` for each `datasets` item
- append bars (rect elements, class `bar bar-{index}`) with the same color inside `.dataset` for each `dataset.data` item
  - bars are distributed within each column enclosed by 2 adjacent grid-x lines  
  - bars in different `.dataset` have unique colors
  - bars in the same column are evenly distributed

Update `index.js`, create `BarChart` instances when `config.type` is `bar`.