I've only been able to test with a MacBook but you are unable to zoom using a trackpad. If you try to zoom using a pinch gesture the graph goes full screen. 

Expected. You should be able to zoom the graph through the same levels as on the graph zoom slide bar, using a two-finger pinch.

Steps to reproduce
* Create a bolt flow macro
* Attach it to a game object as a component
* Open the graph
* Try to zoom in and out using the MacBook trackpad

Notice issue >> There is no zoom. The graph goes full screen.

Note: I've noticed that sometimes it works but is very hit and miss. It seems to be fighting the Unity palette window, which will full size on a pinch zoom. 

I might be worth adding a toggle here. On macOS, the option key + click will let you pan the graph. Perhaps option + pinch to zoom it. 

UPDATE: I've managed to figure it out. The gesture is two-finger on a straight verticle motion up and down. This will zoom in and out. I would rather have modifier key here as going straight up and down is quite tricky.