I have written a QwtPlot on MainWindow,and set it as centreWidget.
Here is the code i wrote based on the Qwt examples:
Qt Code: 
  1. Plot::Plot( QWidget *parent ):
  2.     IncrementalPlot( parent ),
  3.     d_timer( 0 )
  4. {
  5.     enableAxis(xBottom,false);
  6.     enableAxis(yLeft,false);
  7.     setAxisMaxMajor(xBottom,17);
  8.     setAxisMaxMajor(yLeft,10);
  9.  
  10.     d_rescaler = new QwtPlotRescaler( canvas() );
  11.     d_rescaler->setReferenceAxis( QwtPlot::xBottom );
  12.     d_rescaler->setAspectRatio( QwtPlot::yLeft, 1.0 );
  13.     d_rescaler->setAspectRatio( QwtPlot::yRight, 0.0 );
  14.     d_rescaler->setAspectRatio( QwtPlot::xTop, 0.0 );
  15.     d_rescaler->setRescalePolicy( QwtPlotRescaler::Expanding );
  16.     d_rescaler->setEnabled( true );
  17.     d_rescaler->rescale();
  18.  
  19.     plotLayout()->setAlignCanvasToScales( true );
  20.  
  21.     QwtPlotGrid *grid = new QwtPlotGrid;
  22.     grid->setMajorPen( Qt::black, 0, Qt::SolidLine );
  23.     grid->attach( this );	
  24.  
  25.     setCanvasBackground( Qt::gray ); 
  26.  
  27.     setAxisScale( xBottom, 0, 1000 );
  28.     setAxisScale( yLeft, 0, 1000 );
  29.     canvas()->setMouseTracking(true);
  30.     replot();
  31.  
  32. 	// panning with the left mouse button
  33.     new QwtPlotPanner( canvas() );
  34.  
  35. 	// zoom in/out with the wheel
  36.     QwtPlotMagnifier *magnifier = new QwtPlotMagnifier( canvas() );
  37.     magnifier->setMouseButton( Qt::NoButton );
  38.  
  39.     picker = new QwtPlotPicker(QwtPlot::xBottom, QwtPlot::yLeft,QwtPlotPicker::CrossRubberBand, QwtPicker::AlwaysOn,canvas());
  40. }
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I came as:
Square.png

I would like to keep the grid square.

But when i zoomed in,it became like this:
Rectangular.png

I have found some information about this,like the grid is bound to the ScaleTicks.
But i still didn't found the solution.Could anyone help me out?@Uwe?

Thanks first!