title: Matplotlib.colors.ListedColormap in python
body: <pre><code>def plot_decision_regions(X, y, classifier, resolution=0.02):
      # setup marker generator and color map
      markers = ('s', 'x', 'o', '^', 'v')
      colors = ('red', 'blue', 'lightgreen', 'gray', 'cyan')
      cmap = ListedColormap(colors[:len(np.unique(y))])
      # plot the decision surface
      x1_min, x1_max = X[:, 0].min() - 1, X[:, 0].max() + 1
      x2_min, x2_max = X[:, 1].min() - 1, X[:, 1].max() + 1
      xx1, xx2 = np.meshgrid(np.arange(x1_min, x1_max, resolution),
          np.arange(x2_min, x2_max, resolution))
      Z = classifier.predict(np.array([xx1.ravel(), xx2.ravel()]).T)
      Z = Z.reshape(xx1.shape)
      plt.contourf(xx1, xx2, Z, alpha=0.4, cmap=cmap)
      plt.xlim(xx1.min(), xx1.max())
      plt.ylim(xx2.min(), xx2.max())
      # plot class samples
      for idx, cl in enumerate(np.unique(y)):
      plt.scatter(x=X[y == cl, 0], y=X[y == cl, 1],alpha=0.8, 
   c=cmap(idx),marker=markers[idx], label=cl)
  </code></pre>
  
  <p>I was going through perceptron training in machine learning from python machine learning and found this code.In function argument classifier represent perceptron and X is input features y is output vector.I Don't understand what does ListedColormap do ?In addition to that what does meshgrid do ? I am newbie to pandas matplotlib library kindly explain me this code and what we want to do in this code? </p>