[Rd] pretty.Date(): new "halfmonth" time step

[Felix Andrews]

Hi R-devel / R-core

In the new pretty() methods for Date and POSIXct
https://svn.r-project.org/R/trunk/src/library/grDevices/R/prettyDate.R
there is currently a "pretty" time step listed as "15 DSTdays"... but
this actually doesn't line up well with months.

Much better to implement directly what this is trying to do: i.e. to
have a "halfmonth" time step. This is just the union of two "monthly"
sequences, one on the 1st of each month and another on the 15th of
each month.

With this in place we have:

prettyDate(as.Date(c("2002-02-02", "2002-05-01")))
# [1] "2002-02-15" "2002-03-01" "2002-03-15" "2002-04-01" "2002-04-15"
"2002-05-01"

The proposed patch is attached.

Regards
-Felix


-- 
Felix Andrews / 安福立
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