It looks like LabVIEW doesn't give the ability to rotate a graph's cursor label. There's an idea submitted for it, though: https://forums.ni.com/t5/LabVIEW-Idea-Exchange/Rotate-and-format-cursor-labels-on-graphs/idi-p/10110....
You can use a 2-D picture control, write rotated text to it, and move the pic to the location of the cursor. Here's a starting point for you:
You'll have to run with the ball with this code. It'll work as long as the X and Y scales are positive and you use the mouse to drag the cursor around. You'll have to take into account negative coordinates and scaling as the code tries to convert the graph's coordinates into front panel positions programmatically. It gets tricky as graph y coordinates are positive going up while front panel positions are positive going down.