If you can't find a way to do this with a property node - which there doesn't seem to be, one way could be to just have two graphs, one atop the other. Set the formats and then make visible with a property node.
I checked the properties and came up empty myself. I think paul s' answer is going to be the best workaround. As a variation on his theme, you can programatically move the graphs on and off screen when you want to change the format of the marker labels. This would allow for user interaction with the graph that is visible.
