Axis labels are objects. If you already have existing axis labels and enter Button Edit Mode (Edit:Button Edit Mode) you will notice the object name appear above the label. For example the object name for the bottom x axis label is XB. The object name for the left y axis label is YL. Therefore to change the label (or create a new label) in script of the bottom x axis (assuming the graph in question is the active window) you would use (example):
If you know the name of the Graph's window you can use the rotate property of the Layer.Axis.Label object. Try following Labtalk command for rotating y axis label to 90 degrees: