R help - Feb 2010 - Plot questions: grid line do not correspond to tick marks and 2 line axis label

Hello list,

Thank you very much for replying to my earlier questions. I have a few new
questions about R plots:

Example: tempdata:
RDate Price
2009-12-09 12:00 100
2009-12-09 15:00 99
2009-12-09 18:00 102
....
2009-12-10 8:00 120
2009-12-10 10:00 110
2009-12-10 16:00 105

My code: (RDate already in R date-time class)
plot (price~RDate, data=tempdata, "l")
axis.POSIXct(1, at=seq(min(data$RDate), max(data$RDate)+60, by="3
hours"),
format="%H:%M")
grid(lwd=1)

1. The default grid lines do not seem to correspond with the ticks in my
axis. Grid has options for absolute numbers of grid lines, but I would not
want to paste an absolute number here, as I don't know the number of ticks
generated by seq.

2. The tick label on the x axis should be in 2 lines. My x axis is the time
and date (2009/12/09 12:00), I want to show the time on the first line and
some of the dates on the second, visually as below:

-------|----------------|------------------|------------------------
    12:00         15:00         18:00
    12/09                          12/09

How should I specify this?

3. I don't want line to automatically connect the between the missing
periods, i.e., suppose my data was hourly from 12/09 to 12/10, but there is
no data between 2009-12-09 18:00 and 2009-12-10 8:00, the two data points at
these dates should not be connected.

4. I would like to label the uppermost values of the axes, which R does not
seem to label.

5. Is there a graphic package where perhaps all these details have been
figured out?

Thank you very much for your help!

Tam

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