sparkchart.go
func (b *SparkChart) SetTop(top float64) {
	b.mtx.Lock()
	defer b.mtx.Unlock()

	b.topValue = top
}
func (b *SparkChart) SetTop(top float64) {
	b.mtx.Lock()
	defer b.mtx.Unlock()

	b.topValue = top
}
// SetTop sets the theoretical highest value of data flow
// to scale the chart