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Thread: qwtplot drawing lines with negative values

  1. #1
    Join Date
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    Default qwtplot drawing lines with negative values

    Good day to you!
    I'm trying to draw lines on my QwtPlot graph, and using this construction:
    Qt Code:
    1. ui_plot->setAutoFillBackground(true);
    2. ui_plot->setCanvasBackground(QColor(0xE3E3E3));
    3. ui_plot->setAxisScale(QwtPlot::yLeft, -1, 1);
    4. ui_plot->setAxisScale(QwtPlot::xBottom, -1, 1);
    5. ui_plot->enableAxis(QwtPlot::yLeft, true);
    6. ui_plot->enableAxis(QwtPlot::xBottom, true);
    7. QwtPlotCurve *xline = new QwtPlotCurve();
    8. QwtPlotCurve *yline = new QwtPlotCurve();
    9.  
    10. double x[3] ={-1.0, 0.0, 1.0};
    11. double y[3] ={0,0,0};
    12.  
    13. xline->setData(new QwtCPointerData(x,y,(size_t)3));
    14. xline->setPen(QPen(QColor(Qt::black)));
    15. xline->attach(ui_plot);
    16. ui_plot->replot();
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    I expect to have line, drawn from -1 to 1. But for some reason i always receive line from 0 to +∞. It's weird and i really want to know, what kind of data i should use to draw a simple x[-1;0;1] y[0;0;0] line. Can you help me please?

  2. #2
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    Default Re: qwtplot drawing lines with negative values

    Guess x/y are on the stack and get overwritten by some random data.

    Uwe

  3. #3
    Join Date
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    Default Re: qwtplot drawing lines with negative values

    I solved it this way:
    Qt Code:
    1. double x1[3] = {-1.5, 0, 1.5};
    2. double y1[3] = {0, 0, 0};
    3. double x2[3] = {0, 0, 0};
    4. double y2[3] = {-1.5, 0, 1.5};
    5.  
    6. x_1_line->setSamples(x1,y1,3);
    7. y_1_line->setSamples(x2,y2,3);
    8.  
    9. x_1_line->setPen(QPen(QColor(Qt::black),1,Qt::DashLine));
    10. y_1_line->setPen(QPen(QColor(Qt::black),1,Qt::DashLine));
    11.  
    12. x_1_line->attach(ui_plot);
    13. y_1_line->attach(ui_plot);
    14.  
    15. ui_plot->replot();
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