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