グラフの拡大・縮小

#include <qapplication.h>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include <qwt_plot_grid.h>
#include <qwt_symbol.h>
#include <qwt_legend.h>
#include <qwt_plot_magnifier.h>

int main(int argc, char *argv[])
{
    QApplication a( argc, argv );

       QwtPlot plot;
       plot.setTitle( "Plot Demo" );
       plot.setCanvasBackground( Qt::white );
       plot.setAxisScale( QwtPlot::yLeft, 0.0, 10.0 );
       plot.insertLegend( new QwtLegend() );

       QwtPlotGrid *grid = new QwtPlotGrid();
       grid->attach( &plot );

       QwtPlotCurve *curve = new QwtPlotCurve();
       curve->setTitle( "Some Points" );
       curve->setPen( Qt::blue, 4 ),
       curve->setRenderHint( QwtPlotItem::RenderAntialiased, true );

       QwtSymbol *symbol = new QwtSymbol( QwtSymbol::Ellipse,
           QBrush( Qt::yellow ), QPen( Qt::red, 2 ), QSize( 8, 8 ) );
       curve->setSymbol( symbol );

       QPolygonF points;
       points << QPointF( 0.0, 4.4 ) << QPointF( 1.0, 3.0 )
           << QPointF( 2.0, 4.5 ) << QPointF( 3.0, 6.8 )
           << QPointF( 4.0, 7.9 ) << QPointF( 5.0, 7.1 );
       curve->setSamples( points );
       curve->attach( &plot );

       QwtPlotMagnifier* magnifier = new QwtPlotMagnifier( plot.canvas());
       magnifier->setMouseButton(Qt::LeftButton);

       plot.resize( 600, 400 );
       plot.show();

       return a.exec();
}