title: pyplot combine multiple line labels in legend
body: <p>I have data that results in multiple lines being plotted, I want to give these lines a single label in my legend. I think this can be better demonstrated using the example below,</p>
  <pre><code>a = np.array([[ 3.57,  1.76,  7.42,  6.52],
                [ 1.57,  1.2 ,  3.02,  6.88],
                [ 2.23,  4.86,  5.12,  2.81],
                [ 4.48,  1.38,  2.14,  0.86],
                [ 6.68,  1.72,  8.56,  3.23]])
  
  plt.plot(a[:,::2].T, a[:, 1::2].T, 'r', label='data_a')
  
  plt.legend(loc='best')
  </code></pre>
  <p>As you can see at Out[23] the plot resulted in 5 distinct lines. The resulting plot looks like this
  <img src="https://i.stack.imgur.com/YJkI2.png" alt="legend of multiple line plot" /></p>
  <p>Is there any way that I can tell the plot method to avoid multiple labels? I don't want to use custom legend (where you specify the label and the line shape all at once) as much as I can.</p>