We are developing an application that uses a plot to show data. We allow zooming in and out - no scrolling up and down is necessary. 

Double click is used to zoom in, backspace to zoom out. + and - keys are zoom in and out 

We have been unable to standardize on up and down arrows - does an up arrow typically mean zoom in or zoom out?

We are writing the code to do the zoom and it is a UI question - if there is a better site, please migrate.