title: Color is not working in pyqtgraph like the way it works in matplotlib
body: <p>I have a 2d numpy array as following </p>
  
  <pre><code>import numpy as np
  agents = np.array([[  1. , -71.8,  41.2],
         [  1. , -71.8,  41.3],
         [  0. , -71.8,  41.4],
         [  0. , -71.7,  41.4],
         [  0. , -71.6,  41.4]])
  </code></pre>
  
  <p>Now I want to visualize it. In matplotlib i can do something like this as I need different color based on the value of the first column.</p>
  
  <pre><code>import matplotlib.pyplot as plt
  plt.scatter(agents [:,1],agents [:,2],c=agents [:,0])
  </code></pre>
  
  <p>But how can I get the same result in pyqtgraph?
  I tried this </p>
  
  <pre><code>app = QtGui.QApplication(sys.argv)
  mw = QtGui.QMainWindow()
  mw.resize(800, 800)
  view = pg.GraphicsLayoutWidget()
  mw.setCentralWidget(view)
  mw.setWindowTitle('pyqtgraph example: ScatterPlot')
  w1 = view.addPlot()
  x = agents[:,1]
  y = agents[:,2]
  z = agents[:,0]
  s = pg.ScatterPlotItem(x, y,pen=pg.mkPen(z))
  w1.addItem(s)
  mw.show()
  sys.exit(QtGui.QApplication.exec_())
  
  </code></pre>
  
  <p>Got this exception</p>
  
  <pre><code>Exception: Not sure how to make a color from "(array([1., 0., 0., 0.]),)"
  </code></pre>