The reason your scroll bar is disabled is because you are displaying the entire data set. If you were to enter a different start or stop time, the scroll bar would be displayed. To enter a different start or stop time time, just highlight which one you want to change and type in a new value. If you want to do it programatically, turn off autoscale of the X axis and then you can use a property node to set x min and x max. If you want to display data point by point like a chart does, what I would recomend is that you read in the entire data set like you're doing but instead of wiring it directly to the XY Graph, wire it to a for loop with a wait statement inside. There is a shipping example called XY Chart that you should look at. It uses a subVI called XY Chart Buffer that manages the data. If you put the subVI and the graph inside the for loop, you should get what you want.
