XY-graphs do not have an in-built buffer the way charts do (there is no XY-chart) so you will have to hold the data in a buffer/buffers you make yourself. A circular buffer is a good option for this.
Let's say that when adding an input you dynamically create a circular buffer for it and you also make a functional global that administers the buffers (keeps a list of the signals available and which circular VI that holds it's data, in this case the timestamp X and the signal value Y). Now you can let the user see that list and select which ones to show on the XY graph. Every now and then you read that selection list. Wire the selection list to a for loop e.g. with autoindexing and inside it you call the circular buffers holding the data. You bundle the X and Y (or do it inside the buffer) and wire it out of the for loop...now you have an array of XY plots ready to be wired into the XY graph...refresh at a rate high enough to give a "real time" impression for the user.
