qwtplot drawing lines with negative values

[dimbor2708]

Good day to you!
I'm trying to draw lines on my QwtPlot graph, and using this construction:

ui_plot->setAutoFillBackground(true);
	ui_plot->setCanvasBackground(QColor(0xE3E3E3));
	ui_plot->setAxisScale(QwtPlot::yLeft, -1, 1);
	ui_plot->setAxisScale(QwtPlot::xBottom, -1, 1);
	ui_plot->enableAxis(QwtPlot::yLeft, true);
	ui_plot->enableAxis(QwtPlot::xBottom, true);
	QwtPlotCurve *xline = new QwtPlotCurve();
	QwtPlotCurve *yline = new QwtPlotCurve();
 
	double x[3] ={-1.0, 0.0, 1.0};
	double y[3] ={0,0,0};
 
	xline->setData(new QwtCPointerData(x,y,(size_t)3));
	xline->setPen(QPen(QColor(Qt::black)));
	xline->attach(ui_plot);
	ui_plot->replot();

ui_plot->setAutoFillBackground(true);
	ui_plot->setCanvasBackground(QColor(0xE3E3E3));
	ui_plot->setAxisScale(QwtPlot::yLeft, -1, 1);
	ui_plot->setAxisScale(QwtPlot::xBottom, -1, 1);
	ui_plot->enableAxis(QwtPlot::yLeft, true);
	ui_plot->enableAxis(QwtPlot::xBottom, true);
	QwtPlotCurve *xline = new QwtPlotCurve();
	QwtPlotCurve *yline = new QwtPlotCurve();

	double x[3] ={-1.0, 0.0, 1.0};
	double y[3] ={0,0,0};

	xline->setData(new QwtCPointerData(x,y,(size_t)3));
	xline->setPen(QPen(QColor(Qt::black)));
	xline->attach(ui_plot);
	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?