You can zoom into a particular area of your plot by using View - Zoom. After selecting the option, the cursor, when in the main window, changes to indicate that the response viewer is waiting for you to tell it where you want to zoom. Move the cursor to a corner of the region you want magnified and press the left mouse button. Holding it down, drag the cursor to the desired opposite corner of the desired area and release the mouse button. The specified region is expanded to fill the window.
Shortcut Key: Space bar or Middle Mouse button