st: Graph Plots with Date-Time

[drreg@earthlink.net]

Serial number:  30110532867
Licensed to:  Reginald C. Jordan

v.11 updated on WinXP

Here is a partial dataset that I am attempting to work with:

time                   cdconc	ws	wd	t
5/19/2010 13:00 	.077	2	60	24.1
5/19/2010 13:30 	.077	1.9	58	24.5
5/19/2010 14:00 	.006	1.8	46	24.8
5/19/2010 14:30 	.006	1.6	14	25.9
5/19/2010 15:00 	.008	1.8	30	26.2
5/19/2010 15:30 	.008	1.6	33	26.9
5/19/2010 16:00 	.006	1.8	21	27.1
5/19/2010 16:30 	.006	1.6	20	27.3
5/19/2010 17:00 	.007	1.5	35	27.6
5/19/2010 17:30 	.007	1.8	53	27.2
5/19/2010 18:00 	.005	1.6	35	27
5/19/2010 18:30 	.005	1.8	48	26.4
5/19/2010 19:00 	0	1.4	36	25.8
5/19/2010 19:30 	0	1.1	37	24.7
5/19/2010 20:00 	.014	1	31	23.2
5/19/2010 20:30         .014	.9	33	22.2
5/19/2010 21:00         .004	.8	41	21.6
5/19/2010 21:30         .004	.6	35	20.9
5/19/2010 22:00         0	.7	33	20.4
5/19/2010 22:30         0	.7	44	20
5/19/2010 23:00         .007	.5	357	19.2
5/19/2010 23:30         .007	.4	351	19

var       label       type       format
time      Time        str15      %15s
cdconc    Cdconc      float      %9.0g
ws        WS          float      %9.0g
wd        WD          int        %8.0g
t         T           float      %9.0g 

Questions

1. Is there an easy way to drop observations when time is on the half-hour (1:30, 2:30, etc.)?
2. Ultimately, what I want to do is plot cdconc vs time, where time is 12AM, 3AM, 6AM, 9AM, etc. Does time need to be converted from a string variable and date-time formatted first? I've read through Cox,Getting nice time-of-day labels on a graph, SJ, February 2003, revised September 2003, but I don't know exactly how this helps me.
3. In general, do you need to separate date and time before doing this sort of plot (cdconc vs. time)? If you do, how do you format the time part?

Thanks much.

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