Missing import/export declarations for private classes

#include <QtCharts/QChartGlobal>
#include <private/xychart_p.h>
#include <QtCharts/QChart>
#include <QtGui/QPen>

QT_CHARTS_BEGIN_NAMESPACE

class MyLineChartItem :  public XYChart
{
    Q_OBJECT
    Q_INTERFACES(QGraphicsItem)
public:
    explicit MyLineChartItem(MyLineSeries *series, QGraphicsItem *item = 0) {
        domain()->minX(); // undefined reference error
    }
    ~MyLineChartItem() {}
}