Transformation for custom value in SparklinesReferenceLine

[Dem0n3D]

  <Sparklines data={this.props.data}>

        <SparklinesLine />

        <SparklinesReferenceLine type="custom" value={point.y} />

  </Sparklines>

in this PR, point.y will be transformed with dataToPoints (max in min will be calculated automatically in Sparklines).

[MattVoda]

Hey, any progress on this?