title: pyqtgraph: how to drag or drop into plotItems
body: <ol>
  <li><p>I want to drag an item from a QListWidget into pyqtgraph.plotItem. How can I do that.</p></li>
  <li><p>When I click on a plotItem I want to know which item has been clicked.
  I think I need to reimplement some classes' method. Then which class?
  Many thanks.</p></li>
  </ol>