The graph tool's Point of Interest system greatly reduces the number of times you will wish to zoom into a graph but the graph tool supports zooming in a number of ways.
Zooming With The Keyboard
You can zoom in and out using the keyboard.
Page Up - Zooms the axes out
Page Down - Zooms the axes in
Home - Centres the axes on the origin.
Starting Axes
Page Up - Zooms OUT by 33%
Page Down - Zooms IN by 33%
Zooming With The Mouse
Zoom Mode
Before using your mouse to zoom, you must first enter Zoom Mode. Push the 
button on the graph tool's toolbar.
Once in Zoom mode you can use the mouse to Zoom into and out from functions.
Click Zooming
If you click the left mouse button somewhere on the plane, the graph tool will zoom in to the point you click on. This allows you to quickly and easily zoom in on points of interest.
If you right click, the graph tool will zoom out.
Rectangle Zooming
If you sweep out a rectangle while holding down the left mouse button, the graph tool will zoom into the rectangle you have defined.
Undo Zoom
Undo remembers zoom events so you can undo any zoom by choosing Undo from the Edit menu or by typing Ctrl+Z