Well, that fixes the errors in your code, but I don't quite understand your last statements. It seems as if you're trying to build some sort of 2D histogram, but I'm not sure. I don't have whatever device you're getting data from, so I don't know what the data is supposed to look like.
One warning: since you're using the Express Build XY Graph that VI will eat up memory quite quickly with the "Reset Graph" set to false. Also, it will slow down the computer quite quickly. On my system LabVIEW jumped to 100% CPU usage iafter about 1 minute as it was trying to shuffle memory around.