I lost 2 workdays with this exact problem but I have a solution so I'll post it in the only part on the internet this matter is discussed. Even if it's almost 10 years after it was published.
PROBLEM: Labview waveform graph would round cursor on X values like Götz described.
TROUBLESHOOT: First I checked what xseadog proposes, maybe the data doesn't actualy hold these values I'm trying to access: Right click on the graph, export to excel and check it has your value, it did have it in my case but I couldn't access it on the graph wether I manualy moved the cursor or I typed on the cursor box.
SOLUTION: If you want to know the Y value for a certain X your cursor can't acces use the 4 diamond arrows to the left of the cursor box to move through the data, these for some reason can access all of your data points.
