Project: chart
Task (challenging): In `./common/BarChart.js`, when `options.dataLabels` is true:
- append g element (class `dataLabels dataLabels-{index}`) inside `svg` for each `dataset`
- append data labels (each class `dataLabel dataLabel-{index}`) inside `.dataLabels` for each `dataset.data` item
- each dataLabel is above the `.dataset` bars, distributes from left to right
- each dataLabel color is the same as `.dataset` bars color