code: func (p *Panel) AddTarget(t *Target) {
  	switch p.OfType {
  	case GraphType:
  		p.GraphPanel.Targets = append(p.GraphPanel.Targets, *t)
  	case SinglestatType:
  		p.SinglestatPanel.Targets = append(p.SinglestatPanel.Targets, *t)
  	case TableType:
  		p.TablePanel.Targets = append(p.TablePanel.Targets, *t)
  	}
  	// TODO check for existing refID
  }
docs: // AddTarget adds a new target as defined in the argument
  // but with refId letter incremented. Value of refID from
  // the argument will be used only if no target with such
  // value already exists.