You can zoom in and out by scrolling the mouse wheel, by typing ZI or ZO, or by using the + / - keys (you don't have to press Shift to use the + key, so strictly speaking, it is = rather than +!). You can set the zoom to be centred either on the mouse position or on the centre of the display by going to Project /Preferences (or type Ctrl/Cmd ,) and making your selection in the Spectrum tab. Just below this you can set the Manual Zoom, the percentage by which the keyboard shortcuts will zoom the spectrum in or out.
To zoom in on a particular area, hold down Shift while left-dragging the mouse over the area of interest. A yellow box will illustrate the selected area.
Revert to a full spectrum (maximum) zoom level by clicking on the Maximise Zoom button in the toolbar.
The aspect ratio of your spectrum is changed by scrolling the mouse wheel on one of the axes. This selectively zooms that axis, thereby, in effect, changing the aspect ratio. You can lock the aspect ratio by clicking on LOCK in the bottom left-hand corner of the spectrum display. Note that if you have the aspect ratio lock turned on and you then do an area zoom with the mouse, you will be constrained to fit the locked aspect ratio.
You can store a particular zoom level using the Store Zoom button in the toolbar, or typing ZS. To revert to this stored zoom level either press the Restore Zoom button in the toolbar or type ZR.
It is also possible to move to the zoom setting you last used with the Previous Zoom button in the toolbar or by typing ZP. Similarly you can then move forwards to the next zoom level again with Next Zoom or ZN.
The shortcut SZ will bring up a pop-up in which you can set a zoom area to specific ppm limits.