title: QCPItemLine is being paint automatically
body: <p>Im trying using QCustomPlot in my gui</p>
  
  <p>MainWindow.h</p>
  
  <pre><code>public:
  void initArrow();
  
  private:
  QCustomPlot* grid;
  QCPItemLine* arrow;
  </code></pre>
  
  <p>MainWindow.cpp</p>
  
  <pre><code>void MainWindow::initArrow() {
  arrow = new QCPItemLine(ui-&gt;grid);
  QPen pen(Qt::red);
  pen.setWidth(4);
  
  arrow-&gt;setPen(pen);
  arrow-&gt;setHead(QCPLLineEnding::esLineArrow);
  arrow-&gt;setHead(QCPLLineEnding::esDisc);
  }
  </code></pre>
  
  <p>in this situation i'm playing the program, i resize the window and i see an arrow on the plot with coordinates (0,0) and (1,1).
  this is the code
  .
  I didnt use replot!
  I didnt use start!
  I didnt use end!</p>
  
  <p>in addition, when i will understand what is the problem I'm asking what is the correct way to draw a line when i'm trying to draw above other graph.</p>
  
  <p>normally if I'm using points it will be like:</p>
  
  <pre><code>QVector&lt;double&gt; x;
  QVector&lt;double&gt; y;
  ui-&gt;grid-&gt;graph(number)-&gt;setData(x,y);
  ui-&gt;grid-&gt;graph(number)-&gt;replot();
  </code></pre>
  
  <p>but I cant do setData with a line and its not related to some graph so i can i Draw the line on a specefic graph??</p>