This is detailed in the help. You have to do two things to set up the graph.
1) Enable Zooming/Panning
2) Make sure it is in Hot or Normal mode (not Indicator)
Then you can use the mouse to zoom (holding down and left clicking) or pan (holding down and left clicking).
If you look in the online help under "Operating Graph Controls" you will find detailed instructions on this.
Best Regards,
Chris Matthews
National Instruments