title: pyqtgraph: add legend for lines in a plot
body: <p>I'm using pyqtgraph and I'd like to add an item in the legend for InfiniteLines.</p>
  
  <p>I've adapted the example code to demonstrate:</p>
  
  <pre><code># -*- coding: utf-8 -*-
  """
  Demonstrates basic use of LegendItem
  
  """
  import initExample ## Add path to library (just for examples; you do not need this)
  
  import pyqtgraph as pg
  from pyqtgraph.Qt import QtCore, QtGui
  
  plt = pg.plot()
  plt.setWindowTitle('pyqtgraph example: Legend')
  plt.addLegend()
  
  c1 = plt.plot([1,3,2,4], pen='r', name='red plot')
  c2 = plt.plot([2,1,4,3], pen='g', fillLevel=0, fillBrush=(255,255,255,30), name='green plot')
  c3 = plt.addLine(y=4, pen='y')
  # TODO: add legend item indicating "maximum value"
  
  ## Start Qt event loop unless running in interactive mode or using pyside.
  if __name__ == '__main__':
      import sys
      if (sys.flags.interactive != 1) or not hasattr(QtCore, 'PYQT_VERSION'):
          QtGui.QApplication.instance().exec_()
  </code></pre>
  
  <p>What I get as a result is:
  <img src="https://i.stack.imgur.com/5Z73c.png" alt="plot image"></p>
  
  <p>How do I add an appropriate legend item?</p>