How to histogram day-of-week, and have string labels

Question

I have a data-frame of dates (Date object); see bottom. I'm trying to convert them to day-of-week and then draw a histogram, but ideally where the labels are 'Monday'...'Sunday' (not numeric)

I have two distinct problems:

1. It's easy to convert a Date object to day-of-week, but the result is string or numeric, not an object.
2. When I get a histogram, the bins and labels are wrong (see below).

If I use weekdays(dat), the output is string ("Monday"...) which cannot be used in hist().

Alternatively, if I convert to numeric data, how to get string labels on hist()?

> dotw <- with( month.day.year(dat[,1]), day.of.week(month,day,year) )
> hist(xxx,labels=c('M','Tu','W','Th','F','Sa','Su'),col='black') # WTF?!
> hist(dotw,xlab=list('M','Tu','W','Th','F','Sa','Su'))

Does not work as intended for labeling. What's with the 0.5-width bins? And also, how to prevent the lack of gap between Sunday->0 and Monday->1? Ideally, no gaps between columns.

My data looks like:

> dat
  [1] "2010-04-02" "2010-04-06" "2010-04-09" "2010-04-10" "2010-04-14" "2010-04-15" "2010-04-19"
  [8] "2010-04-21" "2010-04-22" "2010-04-23" "2010-04-26" "2010-04-28" "2010-04-29" "2010-04-30"
 ...

> str(dat)
 Date[1:146], format: "2010-04-02" "2010-04-06" "2010-04-09" "2010-04-10" "2010-04-14" "2010-04-15" ...

> str(weekdays(dat))
 chr [1:146] "Friday" "Tuesday" "Friday" "Saturday" "Wednesday" "Thursday" "Monday" ...
> hist(weekdays(dat))
Error in hist.default(weekdays(dat)) : 'x' must be numeric

Answer 1

dat <- as.Date( c("2010-04-02", "2010-04-06", "2010-04-09", "2010-04-10", "2010-04-14", 
       "2010-04-15", "2010-04-19",   "2010-04-21", "2010-04-22", "2010-04-23","2010-04-24", 
        "2010-04-25", "2010-04-26", "2010-04-28", "2010-04-29", "2010-04-30"))
 dwka <- format(dat , "%a")
 dwka
# [1] "Fri" "Tue" "Fri" "Sat" "Wed" "Thu" "Mon"
#  [8] "Wed" "Thu" "Fri" "Sat" "Sun" "Mon" "Wed"
# [15] "Thu" "Fri"
dwkn <- as.numeric( format(dat , "%w") ) # numeric version
hist( dwkn , breaks= -.5+0:7, labels= unique(dwka[order(dwkn)]))

Answer 2

I suspect you want a barplot rather than a histogram. You can use table to count the days.

barplot(table(weekdays(dat)))

Note that by default the days will be sorted alphabetically, so to order it more naturally you will have to reorder the levels in a factor call:

barplot(table(factor(weekdays(dat),levels=c("Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"))))

Answer 3

Convert your weekdays(dat) to a factor (data type for categorical variables), and unclass it (which will convert to integer) for the histogram. There are operations on the factor class which makes it easy to create the custom x-axis.

## days of the week
days <- c('Sun','Mon','Tues','Wed','Thurs','Fri','Sat')

## sample with replacement to generate data for this example
samples <- sample(days,100,replace=TRUE)

## convert to factor
## specify levels to specify the order
samples <- factor(samples,levels=days)

hist(unclass(samples),xaxt="n")
axis(1,at=1:nlevels(samples),lab=levels(samples))
box()