Use it this way:

 import Data.Time
 import Graphics.Gnuplot.Simple

 main =
    plotPath [XTime, XFormat "%m-%d"] $ prepXTime $
       (UTCTime (fromGregorian 2008 01 01)     0, 1.0) :
       (UTCTime (fromGregorian 2008 01 05) 43200, 5.0) :
       (UTCTime (fromGregorian 2008 01 15)     0, 2.5) :
       []