[Gnuplot-info] Plot time related data by hour, not by day

[Steve C. <sc...@ja...>]

Suppose I have data structured like this:

2009-10-01 10:30:14,16,10,0,0.91,0.29,1.82,1.17
2009-10-01 12:30:15,27,17,1,2.13,0.50,3.52,2.58
2009-10-01 13:30:17,18,13,0,0.76,0.38,2.07,0.88
2009-10-01 14:30:15,22,14,1,1.03,0.32,2.09,1.26
2009-10-01 15:30:16,17,11,1,0.95,0.42,2.26,1.11
2009-10-01 16:30:16,12,9,0,0.74,0.36,2.05,0.83
2009-10-01 17:30:14,14,11,0,0.69,0.24,1.65,0.88
2009-10-01 18:30:17,18,15,0,0.80,0.40,2.08,0.93
2009-10-01 19:30:15,16,12,0,0.76,0.25,1.69,0.94
2009-10-01 20:30:16,14,5,0,0.70,0.19,1.62,0.79
2009-10-01 21:30:15,10,7,0,0.44,0.17,1.25,0.55
2009-10-01 22:30:15,7,6,0,0.41,0.14,1.30,0.44
2009-10-01 23:30:14,4,3,0,0.15,0.09,0.76,0.16
2009-10-02 00:30:20,3,2,0,0.15,0.22,0.89,0.08
2009-10-02 01:30:14,1,1,0,0.07,0.06,0.53,0.08
2009-10-02 02:30:16,2,1,0,3.81,0.98,11.47,0.15
2009-10-02 03:30:14,1,1,0,0.07,0.07,0.57,0.03
2009-10-02 04:30:14,0,0,0,0.03,0.06,0.35,0.02
2009-10-02 05:30:13,1,1,0,0.13,0.08,0.62,0.14
2009-10-02 06:30:12,1,0,0,0.13,0.08,0.57,0.14
2009-10-02 07:30:13,2,2,0,0.23,0.09,0.81,0.26

What I want is a plot where the X-axis represents the TIME OF DAY only, 
without reference to the date, and the Y axes represent various 
measurements I want to
at that time over a series of days.  How do I tell Gnuplot to IGNORE the 
Date portion of the time and only pay attention to the time portion?  In 
fact, I would not mind losing the minutes and seconds either.  Thus 
there would be a series of points representing ta y-value at at 1 AM, 2 
AM, 3 AM, etc. 

Thanks.