increase the size of my plot

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Hello world,

i want to increase the size of my plot but it don't work, i have put this code:

window1= new QWidget(this);
myPlot1=new QwtPlot(window1);
myPlot1->setMaximumWidth(200);

window1= new QWidget(this);
myPlot1=new QwtPlot(window1);
myPlot1->setMaximumWidth(200);

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but it don't change the size of my plot

i have try too:

myPlot1=new QwtPlot(this);
    myPlot1->minimumSize(2000);

myPlot1=new QwtPlot(this);
    myPlot1->minimumSize(2000);

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but it don't increase the size