can re-center when scaling mapCanvas extent

python/gui/qgsmapcanvas.sip

@@ -238,7 +238,8 @@ class QgsMapCanvas : QGraphicsView
     void zoomScale( double scale );
 
     //! Zoom with the factor supplied. Factor > 1 zooms out, interval (0,1) zooms in
-    void zoomByFactor( double scaleFactor );
+    //! If point is given, re-center on it
+    void zoomByFactor( double scaleFactor, const QgsPoint *center = 0 );
 
     //! Zooms in/out with a given center
     void zoomWithCenter( int x, int y, bool zoomIn );

src/gui/qgsmapcanvas.cpp

@@ -1815,10 +1815,10 @@ void QgsMapCanvas::getDatumTransformInfo( const QgsMapLayer* ml, const QString&
   }
 }
 
-void QgsMapCanvas::zoomByFactor( double scaleFactor )
+void QgsMapCanvas::zoomByFactor( double scaleFactor , const QgsPoint* center )
 {
   QgsRectangle r = mapSettings().visibleExtent();
-  r.scale( scaleFactor );
+  r.scale( scaleFactor, center );
   setExtent( r );
   refresh();
 }

src/gui/qgsmapcanvas.h

@@ -307,7 +307,8 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
     void zoomScale( double scale );
 
     //! Zoom with the factor supplied. Factor > 1 zooms out, interval (0,1) zooms in
-    void zoomByFactor( double scaleFactor );
+    //! If point is given, re-center on it
+    void zoomByFactor( double scaleFactor, const QgsPoint *center = 0 );
 
     //! Zooms in/out with a given center
     void zoomWithCenter( int x, int y, bool zoomIn );