[R] formatting dates in axis labels (ggplot2)

[Michael Friendly]

hadley wickham wrote:
> Hi Michael,
> 
>> > I'm having trouble figuring out how to format Date variables when used as
>> > axis labels in graphs.
>> > The particular case here is an attempt to re-create Nightingale's coxcomb
>> > graph with ggplot2,
>> > where I'd like the months to be labeled as "Mar 1885", "Apr 1885", using a
>> > date format
>> > of "%b %Y" applied to label the dates, or really anything other than
>> > "1885-03-01". I know the solution has to do with formatting the dates, while
>> > preserving their
>> > status as an ordered factor, but I don't know how to do that.
> 
> Dates are, by their very nature, ordered so you don't need to keep
> them as an ordered factor.  If you don't do that you'll find that the
> scale_date formatting options will work.

I would have thought so, but I get an error unless I use 
aes(x=factor(Date)...) or aes(x=ordered(Date)...) in the original
call to ggplot:

 > cxc1 <- ggplot(Night1, aes(x = Date, y=Deaths, fill = Cause)) +
+ # do it as a stacked bar chart first
+ geom_bar(width = 1, position="identity", color="black") +
+ # set scale so area ~ Deaths
+ scale_y_sqrt()
 > # A coxcomb plot = bar chart + polar coordinates
 > cxc1 + coord_polar(start=3*pi/2) + opts(title="Causes of Mortality in 
the Army in the East") + xlab("")
stat_bin: binwidth defaulted to range/30. Use 'binwidth = x' to adjust this.
Error in pmin(y, 0) : object 'y' not found

I apologize for including attachments, but I can't get to a server where
I can place the results as links.
If I use aes(x=factor(Date)...), I get an appropriate graph, except that
the dates appear as "1885-03-01". [See attached: night1-factor.png]

With aes(x=ordered(Date)...), the same code gives only a bar chart
[night1-ordered.png]

If I format the date first, I get a result where the dates are ordered
alphabetically [night1-formatted.png]

Night1$dt1 <- format(Night1$Date, "%b %Y")
cxc1 <- ggplot(Night1, aes(x = factor(dt1), y=Deaths, fill = Cause)) +
geom_bar(width = 1, position="identity", color="black") +
scale_y_sqrt()
cxc1 + coord_polar(start=3*pi/2) + opts(title="Causes of Mortality in 
the Army in the East") + xlab("")

For completeness, here is the data I was using:

Night1 <-
structure(list(Date = structure(c(-42278, -42248, -42217, -42187,
-42156, -42125, -42095, -42064, -42034, -42003, -41972, -42278,
-42248, -42217, -42187, -42156, -42125, -42095, -42064, -42034,
-42003, -41972, -42278, -42248, -42217, -42187, -42156, -42125,
-42095, -42064, -42034, -42003, -41972), class = "Date"), Cause = 
c("Disease",
"Disease", "Disease", "Disease", "Disease", "Disease", "Disease",
"Disease", "Disease", "Disease", "Disease", "Wounds", "Wounds",
"Wounds", "Wounds", "Wounds", "Wounds", "Wounds", "Wounds", "Wounds",
"Wounds", "Wounds", "Other", "Other", "Other", "Other", "Other",
"Other", "Other", "Other", "Other", "Other", "Other"), Deaths = c(1.4,
6.2, 4.7, 150, 328.5, 312.2, 197, 340.6, 631.5, 1022.8, 822.8,
0, 0, 0, 0, 0.4, 32.1, 51.7, 115.8, 41.7, 30.7, 16.3, 7, 4.6,
2.5, 9.6, 11.9, 27.7, 50.1, 42.8, 48, 120, 140.1), Regime = structure(c(1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L
), .Label = c("Before", "After"), class = c("ordered", "factor"
))), .Names = c("Date", "Cause", "Deaths", "Regime"), row.names = c(1L,
2L, 3L, 4L, 5L, 6L, 7L, 8L, 9L, 10L, 11L, 25L, 26L, 27L, 28L,
29L, 30L, 31L, 32L, 33L, 34L, 35L, 49L, 50L, 51L, 52L, 53L, 54L,
55L, 56L, 57L, 58L, 59L), class = "data.frame")

-Michael


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