title: QCustomPlot QCPItemLine
body: <p>I want to ask a question about qcustomplot. How can i change itemLine position according to Slider? (like x = a)</p>
  
  <pre><code>#include "itemline.h"
  #include "ui_itemline.h"
  #include "qcustomplot.h"
  
  itemLine::itemLine(QWidget *parent) :
      QMainWindow(parent),
      ui(new Ui::itemLine)
  {
      ui-&gt;setupUi(this);
  
      QCPItemLine *item = new QCPItemLine(ui-&gt;customPlot);
      ui-&gt;customPlot-&gt;addItem(item);
      item-&gt;setPen(QPen(Qt::red));
      item-&gt;start-&gt;setCoords(1,0);
      item-&gt;end-&gt;setCoords(1,5);
  
      connect(ui-&gt;horizontalSlider, SIGNAL(valueChanged(int)), this, SLOT(horzSliderChanged(int)));
  }
  
  itemLine::~itemLine()
  {
      delete ui;
  }
  
  void itemLine::horzSliderChanged(int value)
  {
       // how can i change item position acording to horizontalSlider, like "x = a" line ?
  }
  </code></pre>