Which program produces this strange plot: multiple y values for single x
===

I got the plot from a dissertation.  It is a time series of wind speed.  Time is in x-axis, wind speed is in y-axis. The strange thing is for each value of x (time), there are multiple values of y (wind speed).  I have no idea how the plot was produced.  If someone know, please enlighten me.
Thanks a lot.

[Strange Plot][1]


  [1]: http://i.imgur.com/vBoXE.jpg