[R] Plotting zoo objects with chron axis

[Enrico Schumann]

On Fri, 09 Aug 2013, Manta <mantino84 at libero.it> writes:

> Dear all,
>
> I have a problem which I'm not to fix. I have the following two series:
>
> a=structure(c(33242.5196150509, 34905.8434338503, 38490.6957848689, 
> 38747.0287172129, 38919.1028597142, 39026.3956586941, 38705.5344288997, 
> 38545.6274379387, 38651.2079354205, 38748.2769580121), index =
> structure(c(14029, 
> 14032, 14033, 14034, 14035, 14036, 14039, 14040, 14041, 14042
> ), class = "Date"), class = "zoo")
>
> b=structure(c(53337.7643740991, 52210.2079727035, 50235.4480363949, 
> 50667.1147389469, 50796.5403152116, 51113.5420947436, 51003.3603311344, 
> 50654.0539778796, 49927.5267060329, 49320.1813921822), index =
> structure(c(14029, 
> 14032, 14033, 14034, 14035, 14036, 14039, 14040, 14041, 14042
> ), class = "Date"), class = "zoo")
>
> I want to plot them on the same chart, with the x-axis the Date, and the
> y-axis the time in format %H:%M. At the moment, the y series is expressed in
> seconds from midnight. I am confused with the as.POSIXct conversion.
>
>> hours(min(rollrep))
> [1] 9
>> minutes(min(rollrep))
> [1] 34
>> seconds(min(rollrep))
> [1] 27
>
> however, by doing the following it seems the minimum time is 11:09, which is
> not true.
>
> as.POSIXct(round(min(rollrep)),origin=Sys.Date())
> [1] "2013-08-09 11:09:14 CEST"
>
> Do you have any advice? Is there a way to plot only the hours and minutes,
> without having to go to the Sys.Date?
>
> Thank you,
> Marco

Please provide a *reproducible* example.  (What is 'rollrep'?  'Seconds
from midnight' may be ambiguous when there was a change to/from
summertime or when time zones are relevant.)

Perhaps something like this:

  require("zoo")
  plot(merge(a, b), plot.type = "single", yaxt = "n")
  midnight <- ISOdatetime(2013, 1, 1, 0, 0, 0, "GMT")
  sq <- seq(0, 86400, by = 300)
  axis(2, at = sq, labels = format(midnight + sq, "%H:%M"))


-- 
Enrico Schumann
Lucerne, Switzerland
http://enricoschumann.net