It's not a problem also.
If you need to change this label only once - then simply select its text and type in anything you want - just the same way as with any other text labels.
If you want to change the label in runtime then you must use property nodes - create a propery node for your graph, then select properties->Y-scale->label text (not sure how its spells in english version) and supply new label text value (do not forget to change the property node mode to "write").See the example attached of how this can be done.