Project: chart
Task (challenging): In `./common/LineChart.js`:
- append an initially hidden g element (class `tooltips hidden`) inside `svg`
- when mouse hovers over the `.datasets` area in `svg`, display `.tooltips` near the mouse position
  - select the `.grid-x` (class `selected`) that is horizontally closest to the mouse 
  - append rect element (class `bg`) inside `.tooltips`
  - in `.tooltips`, show all data items from the `.dataset` that corresponds to the selected `.grid-x`
- when mouse not hovers over `.datasets` area in `svg`, hide `.tooltips` (add class `hidden`)