Incorrect labels when plotting xts with yearqtr index class

[joshuaulrich]

The following example shows that axTicksByTime does not create a reasonable format for quarterly data when the index class of the xts object is yearqtr.

require(quantmod)
getSymbols("GS")
GSyq = to.quarterly(Ad(GS), OHLC=FALSE)
periodicity(GSyq)
# Quarterly periodicity from 2007 Q1 to 2015 Q2
axTicksByTime(GSyq)[1:5]
# %n%b%n2007 %n%b%n2008 %n%b%n2010 %n%b%n2011 %n%b%n2013 
#          1          7         13         19         25
GSdt = to.quarterly(Ad(GS), OHLC=FALSE, indexAt="Date")
periodicity(GSdt)
# Quarterly periodicity from 2007-03-30 to 2015-04-10 
axTicksByTime(GSdt)[1:5]
# \nMar\n2007 \nSep\n2008 \nMar\n2010 \nSep\n2011 \nMar\n2013 
#           1           7          13          19          25

This is because format.yearqtr does not support the "%b" format.

format(start(GSyq), "%b %Y")
# [1] "%b 2007"
format(start(GSdt), "%b %Y")
# [1] "Mar 2007"

[joshuaulrich]

After discussing with the zoo team, the best solution is to special-case the yearqtr format.

format.yearmon internally coerces to Date and therefore uses the format.Date method. But format.yearqtr tries to provide something better suited for quarters. If "%b" were supported, then you might want to have left-aligned, centered, right-aligned "%b" versions etc. This would be true for all other higher-resolution formats, not just "%b".

Plus, format.yearqtr behaves consistently with other base R format methods, e.g. format.Date:

R> format(as.yearqtr(Sys.Date()), "%Y Q%q")
[1] "2015 Q2"
R> format(Sys.Date(), "%Y Q%q")
[1] "2015 Q%q"