barchart.go
func (b *BarChart) OnDrawCell(fn func(*BarDataCell)) {
	b.mtx.Lock()
	defer b.mtx.Unlock()

	b.onDrawCell = fn
}
func (b *BarChart) OnDrawCell(fn func(*BarDataCell)) {
	b.mtx.Lock()
	defer b.mtx.Unlock()

	b.onDrawCell = fn
}
// OnDrawCell sets callback that allows to draw multicolored
// bars. BarChart sends the current attrubutes and rune that
// it is going to use to display as well as the current value
// of the bar. A user can change the values of BarDataCell
// depending on some external data or calculations - only
// changing colors and rune makes sense. Changing anything else
// does not affect the chart