you should read in the file first. From the Read File VI, you now have (for example) an orange wire representing an array of doubles. Now take the new data you want to plot, and do a Build Array with the old data (read from the file) and then wire the built arrays into a graph.
Somethings to keep in mind is that the arrays should be the same size (dimension). If you do a Build Array with array 1 having 10 pts, and array 2 having 12 points, the resulting array will be a 2x12 array. Therefore, you will have 2 extra points in array 1's slot in the new 2D array (that are the default vaue for that datatype). If your array are not the same size, you should resize them yourself before you call BUild Array (use Delete From Array, or Array Subset). Another thing to keep in mind is the time base, if the two signals don't have the same time base you can use Align Waveform Timestamps (if you have waveform data), or Interpolate 1D Array if you have an array.
