ENH: made example/svm/plot_iris.py clearer

ogrisel · ogrisel · commit bf1635d72c99 · 2014-01-17T11:21:21.000+01:00
diff --git a/examples/svm/plot_iris.py b/examples/svm/plot_iris.py
@@ -3,31 +3,57 @@
 Plot different SVM classifiers in the iris dataset
 ==================================================
 
-Comparison of different linear SVM classifiers on the iris dataset. It
-will plot the decision surface for four different SVM classifiers.
+Comparison of different linear SVM classifiers on a 2D projection of the iris
+dataset. We only consider the first 2 features of this dataset:
+
+- Sepal length
+- Sepal width
+
+This example shows how to plot the decision surface for four SVM classifiers
+with different kernels.
+
+The linear models ``LinearSVC()`` and ``SVC(kernel='linear')`` yield slightly
+different decision boundaries. This can be a consequence of the following
+differences:
+
+- ``LinearSVC`` minimizes the squared hinge loss while ``SVC`` minimizes the
+  regular hinge loss.
+
+- ``LinearSVC`` uses the One-vs-All (also known as One-vs-Rest) multiclass
+  reduction while ``SVC`` uses the One-vs-One multiclass reduction.
+
+Both linear models have linear decision boundaries (intersecting hyperplanes)
+while the non-linear kernel models (polynomial or Gaussian RBF) have more
+flexible non-linear decision boundaries with shapes that depend on the kind of
+kernel and its parameters.
+
+.. NOTE:: while plotting the decision function of classifiers for toy 2D
+   datasets can help get an intuitive understanding of their respective
+   expressive power, be aware that those intuitions don't always generalize to
+   more realistic high-dimensional problem.
 
 """
 print(__doc__)
 
 import numpy as np
-import pylab as pl
+import matplotlib.pyplot as plt
 from sklearn import svm, datasets
 
 # import some data to play with
 iris = datasets.load_iris()
 X = iris.data[:, :2]  # we only take the first two features. We could
                       # avoid this ugly slicing by using a two-dim dataset
-Y = iris.target
+y = iris.target
 
 h = .02  # step size in the mesh
 
 # we create an instance of SVM and fit out data. We do not scale our
 # data since we want to plot the support vectors
 C = 1.0  # SVM regularization parameter
-svc = svm.SVC(kernel='linear', C=C).fit(X, Y)
-rbf_svc = svm.SVC(kernel='rbf', gamma=0.7, C=C).fit(X, Y)
-poly_svc = svm.SVC(kernel='poly', degree=3, C=C).fit(X, Y)
-lin_svc = svm.LinearSVC(C=C).fit(X, Y)
+svc = svm.SVC(kernel='linear', C=C).fit(X, y)
+rbf_svc = svm.SVC(kernel='rbf', gamma=0.7, C=C).fit(X, y)
+poly_svc = svm.SVC(kernel='poly', degree=3, C=C).fit(X, y)
+lin_svc = svm.LinearSVC(C=C).fit(X, y)
 
 # create a mesh to plot in
 x_min, x_max = X[:, 0].min() - 1, X[:, 0].max() + 1
@@ -37,31 +63,31 @@
 
 # title for the plots
 titles = ['SVC with linear kernel',
+          'LinearSVC (linear kernel)',
           'SVC with RBF kernel',
-          'SVC with polynomial (degree 3) kernel',
-          'LinearSVC (linear kernel)']
+          'SVC with polynomial (degree 3) kernel']
 
 
-for i, clf in enumerate((svc, rbf_svc, poly_svc, lin_svc)):
+for i, clf in enumerate((svc, lin_svc, rbf_svc, poly_svc)):
     # Plot the decision boundary. For that, we will assign a color to each
     # point in the mesh [x_min, m_max]x[y_min, y_max].
-    pl.subplot(2, 2, i + 1)
-    pl.subplots_adjust(wspace=0.4, hspace=0.4)
+    plt.subplot(2, 2, i + 1)
+    plt.subplots_adjust(wspace=0.4, hspace=0.4)
 
     Z = clf.predict(np.c_[xx.ravel(), yy.ravel()])
 
     # Put the result into a color plot
     Z = Z.reshape(xx.shape)
-    pl.contourf(xx, yy, Z, cmap=pl.cm.Paired)
+    plt.contourf(xx, yy, Z, cmap=plt.cm.Paired, alpha=0.8)
 
     # Plot also the training points
-    pl.scatter(X[:, 0], X[:, 1], c=Y, cmap=pl.cm.Paired)
-    pl.xlabel('Sepal length')
-    pl.ylabel('Sepal width')
-    pl.xlim(xx.min(), xx.max())
-    pl.ylim(yy.min(), yy.max())
-    pl.xticks(())
-    pl.yticks(())
-    pl.title(titles[i])
-
-pl.show()
+    plt.scatter(X[:, 0], X[:, 1], c=y, cmap=plt.cm.Paired)
+    plt.xlabel('Sepal length')
+    plt.ylabel('Sepal width')
+    plt.xlim(xx.min(), xx.max())
+    plt.ylim(yy.min(), yy.max())
+    plt.xticks(())
+    plt.yticks(())
+    plt.title(titles[i])
+
+plt.show()
