Anne,
If you are only looking for a simple way to plot a threshold on your graph, then I might suggest creating/using cursors. If you create a waveform graph, you can right click on it to show the cursor legend. From this legend you can create several cursor, name the cursors, and define line styles. You can then gain programmatic control of the cursors using property nodes of the graph. You can set the "threshold" by first making the cursor you wish to change active then specifying a Y value of the cursor's coordinate. You make a cursor active by giving an index arguement to the Properties>>ActiveCursor property. The value can be found in the Property>>Cursor>>Cursor Position>>Cursor Y. If you are given threshold values from a previous vi, then you can use these ar
guements to specify a given Y coordinate of the cursor. The result will be a cursor drawn at the specific threshold on your graph. You can control what type of plot the cursor will have from the Cursor Legend.
-Ryan