You could build the X array the same way you build the Y array. Keep the previous value of X in a shift register and add 1 every time you put an element in Y.
If X is always going to be the reading number, then you could use a waveform graph and just feed Y to it. The only issue you would have is that the first element will have x = 0 rather than 1. The default condition for a graph is to use the index values of the Y array for the X-axis.
If you need the X-axis to start at 1, you have other possibilities.
1. Place a dummy element at location zero in the Y array. Turn off Autoscaling for the X axis. Set the X minimum value = 1.
2. Use Ramp Pattern.vi to generate the X array. (X-Y Graph).
3. Use a for loop and autoindex "i" to the X array. Add 1 to the array. (X-Y Graph).