DOC improving plot_digits_pipe.py by adjusting parameters and making the graph more clear (scikit-learn#14346)

* changed hyperparameter n_iter from 20 to 10

* hyperparameter tuning

* REV Revert

* improved graph

* Review comments

Author: Timsaur

examples/compose/plot_digits_pipe.py

@@ -42,8 +42,8 @@
 
 # Parameters of pipelines can be set using ‘__’ separated parameter names:
 param_grid = {
-    'pca__n_components': [5, 20, 30, 40, 50, 64],
-    'logistic__C': np.logspace(-4, 4, 5),
+    'pca__n_components': [5, 15, 30, 45, 64],
+    'logistic__C': np.logspace(-4, 4, 4),
 }
 search = GridSearchCV(pipe, param_grid, n_jobs=-1)
 search.fit(X_digits, y_digits)
@@ -54,8 +54,9 @@
 pca.fit(X_digits)
 
 fig, (ax0, ax1) = plt.subplots(nrows=2, sharex=True, figsize=(6, 6))
-ax0.plot(pca.explained_variance_ratio_, linewidth=2)
-ax0.set_ylabel('PCA explained variance')
+ax0.plot(np.arange(1, pca.n_components_ + 1),
+         pca.explained_variance_ratio_, '+', linewidth=2)
+ax0.set_ylabel('PCA explained variance ratio')
 
 ax0.axvline(search.best_estimator_.named_steps['pca'].n_components,
             linestyle=':', label='n_components chosen')
@@ -72,5 +73,7 @@
 ax1.set_ylabel('Classification accuracy (val)')
 ax1.set_xlabel('n_components')
 
+plt.xlim(-1, 70)
+
 plt.tight_layout()
 plt.show()