Project: chart
Task (challenging): -In `./common/StepChart.js`, class `StepChart` inherits from `LineChart`, `StepChart`:
  - replace all polylines with step style polylines inside `.datasets`
  - in each step style polyline:
    - each `dataset.data` responds to a horizontal line segment whose width is the distance between two near grid-x
      - the first and the last line segment width is the half
    - connect all line segments to a step style polyline
- Select the option `Step Chart` of `#type` to create `StepChart` instance