Lt spice reference signal for plotting
This should work. Use this type of generator V2: -



PWL means piecewise linear function. In other words it generates a signal based on data in a file but, first you must export the data. This site says: -


  To export waveform data to an ACSII text file:

    Click to select the waveform viewer
    Choose Export from the File menu.
    Select the traces you want exported.



And...


  To import waveform data into LTspice IV you must attach a text file as
  a piecewise linear (PWL) function in a voltage or current source.


EDITed so that Ali Chen can remove his upvote. There's a point here about not being too hasty with upvotes especially when I advised "read the link".