problem with dot style

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hello world,

recently i make a graphic with line style and after this i have changed the style with this command :

mycurve->attach( myPlot1 );
mycurve->setStyle( QwtPlotCurve::Dots );
mycurve->setPen( QPen( Qt::blue, 0.1 ) );

mycurve->attach( myPlot1 );
mycurve->setStyle( QwtPlotCurve::Dots );
mycurve->setPen( QPen( Qt::blue, 0.1 ) );

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and to plot:

QVector<double> x3(0);      
QVector<double> y3(0);
x3.reserve(timeNb3);
y3.reserve(timeNb3);
x3 << Points3_OneXtwoY[i][z][0] ;
y3 << Points3_OneXtwoY[i][z][1] ;
}
mycurve->setSamples(x3.data(),y3.data(),x3.size());
myPlot3->replot();
repaint();

QVector<double> x3(0);      
QVector<double> y3(0);
x3.reserve(timeNb3);
y3.reserve(timeNb3);
x3 << Points3_OneXtwoY[i][z][0] ;
y3 << Points3_OneXtwoY[i][z][1] ;
}
mycurve->setSamples(x3.data(),y3.data(),x3.size());
myPlot3->replot();
repaint();

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all worked nice, but now i re-test this code but it don't plot with dots

i have forget something but i don't understand what

can you help me please ?