Project: svg-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

Update `index.js`, create `StepChart` instances when `config.type` is `step`.