title: Plotting sum of curves in pyqtgraph
body: <p>I want to plot a graph with multiple curves(although in this particular example all plots are linear), and then plot the sum of the graphs as an additional plot.<br>
  I was wondering whether there was a built in way to do it, rather than calculating for each new (x,y) point the coordinates of all other curves at that point and summing them up.<br>
  Here's a quick example(it runs as a standalone script):</p>
  
  <pre><code>from pyqtgraph.Qt import QtGui, QtCore
  import pyqtgraph as pg
  
  app = QtGui.QApplication([])
  win = pg.GraphicsWindow(title="For Test")
  plot = win.addPlot(title='Test')
  
  #First plot:
  x = [1,3,5,7]
  y = [1,2,1,2]
  curve1 = plot.plot(x=x, y=y, pen='r')
  #Second plot
  x = [2,4,6,8]
  y = [0.5,4,2,2]
  curve2 = plot.plot(x=x, y=y, pen='g')
  
  if __name__ == '__main__':
      app.exec_()
  </code></pre>
  
  <p>Expected result is something like:<br>
  x = [1, 2, 3, 4, 5, 6, 7, 8]<br>
  y = [1, ~2, ~4, ~5.5, ~8, ~3.5, 4, 2]<br>
  This problem gets a bit harder if there are actual curves. So is there a built in way to do it?</p>