@observer class Chart(props) {
  @computed get data() { return getDataWithinRange(this.props.dateRange) }
  @computed get dimensions() { return getDimensions() }
  @computed get xScale() { return getXScale() }
  @computed get yScale() { return getYScale() }
  render() {
    // use this.xScale, this.yScale etc. component will re-render automatically if any of those update.
    return <svg className="Chart" />
}