Dear Chilly Charly: Thank you a lot for your point. Actually, I tried it before I posted my question. It did not work in my case. When you look at the legend bar in the right of the graph, you can see one black line in the top of the legend and one black line in the bottom. I click anywhere above the top line or below the bottom line, I can change the color, but the change only apply to the maximum and the minimum color. For example, if you look at the bottom graph and change the color, the value between the maximum and minimum still varies in blue. I think I miss some point here but just can't figure out what I did is wrong. Can you please see my code again and help me figure out what my mistake is? By the way, my code is in LV 8.2, so probably it is why you get a corrupted top graph. I copied the top graph from other colleague's code to get gray level color, but we just can't figure out why I got blue color and why the top color varies in a reversed way.
Yes life is tough. Thank you again for your advice to make life easier.