Ok, i was trying to determine if you were drawing the graph yourself, which you are. You can use transformations on a graphics object to acheive a zoom effect.
Firstly, you need to determine the offest to centre on the area you want to zoom in on and use a TranslateTransform to move the graphics object, then use a ScaleTransform to increase/decrease teh size of the graphics object.
Theres are some great tips on GDI+ at bobpowell.net that you could scan through.