I think you'd probably have to use two distinct axes to do that. You can position them right next to each other and remove the y axis ticks and line from the right-hand one, etc and use linkaxes in the y direction if you want to allow zooming.
Alternatively you could fake it by changing the axis tick labels and plotting the 2nd part from 800, but labelling its ticks as you want them, but that is not a great solution if you need to interact with the graph in any way with e.g. data cursor.
