It kind of seems that what is getting written to the graph is a single element of the array. This would account for the Y axis change and a single point is often hard to see on the graph. Could you post your VI (or a small section that demonstrates the problem)? Since no one else has your hardware, it would help if you created an indicator on the output of the VISA read, run the VI, do a set current values to default, and save the VI before posting it. That way, someone can run the VI with typical data but no hardware.