Hi Adeline,
if you are using a 1-D line fitting algortihm, then it should be asking for two 1-D arrays. One is the X-array, and the other the Y-Array. If you have a single set of data which is equidistant, beginning with zero, then you can "manufacture" an X-Array with a simple for-loop.
Since it sounde like you're plotting on an XY graph, you should already have X- and Y-Values in your code. Try connecting them both up as 1-D arrays. If this doesn't work, look for a 1-D line-fitting VI.
Hope this helps
Shane.
Using LV 6.1 and 8.2.1 on W2k (SP4) and WXP (SP2)