An xy graph will plot exactly the number of points that are in the x and y array. That's assuming that you have the x scale set to autoscale. And, by default, each time the function is called, the graph will reset. You can select properties and change this. You don't mention where you're getting the data. Stick a probe on the input to the express VI and check the size. You can also provide some details on what you're doing or post your VI for someone to look at.