Recalculating ticks/labels after using expand on an axis

[Bob]

Hi all,

How do I get the ticks and tick labels to recalculate after using
expand to "stretch" the y-axis? I am using facet_wrap with free y
scales, so I can't just use ylim to increase the y-axis limits.

economics$RandFactor <- c("FactorA","FactorB")
p <- ggplot(economics, aes(x=date, y=psavert))
p + geom_point() + scale_y_continuous(expand=c(0.05,0)) + facet_wrap(~
RandFactor, ncol=1, scales="free_y")

If I feel that limiting the plot to the range of the data
"unnecessarily" magnifies fluctuations due to domain knowledge, I can
use the expand function to broaden the y-axis limits. However, the
ticks and tick-labels don't seem to recalculate. To take an extreme
case, let's try it with:

p + geom_point() + scale_y_continuous(expand=c(0.05,0)) + facet_wrap(~
RandFactor, ncol=1, scales="free_y")

I get all the tick labels in the center of the plot. How do I get them
to recalculate, so they span the whole (expanded) y-axis?

Many thanks,
Bob

P.S. I understand from the ggplot2 website that the two numbers are a
multiplicative factor and an additive one - does that mean that the
default expand=c(0.05, 0) means 5% padding of the range on each side?
c(1,0) means doubling of the range on both sides (so it's actually
three times the y-range as the original unmodified plot)? And c(0,1)
means increasing the scale by 1 on each side (in the same units as the
data)?

[Dennis Murphy]

Hi:

When I run your example, I get the plot attached to this mail. I don't see any obvious scaling problems. If yours isn't the same, which versions of ggplot2 and plyr do you have? [I don't know if the plot will get through to the list, but it should to the OP.]

> sessionInfo()
R version 2.11.1 (2010-05-31)
x86_64-pc-mingw32

locale:
[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252  
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                         
[5] LC_TIME=English_United States.1252   

attached base packages:
[1] grid      stats     graphics  grDevices utils     datasets  methods 
[8] base    

other attached packages:
[1] ggplot2_0.8.8 proto_0.3-8   reshape_0.8.3 plyr_1.0.3  

loaded via a namespace (and not attached):
[1] tools_2.11.1

Dennis

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[Bob Chan]

Here is my session info. Looks like all the same version numbers.

> sessionInfo()
R version 2.11.1 (2010-05-31)

i386-pc-mingw32

locale:
[1] LC_COLLATE=English_United Kingdom.1252 LC_CTYPE=English_United
Kingdom.1252 LC_MONETARY=English_United Kingdom.1252
[4] LC_NUMERIC=C LC_TIME=English_United
Kingdom.1252

attached base packages:
[1] grid stats graphics grDevices utils datasets

methods base

other attached packages:
[1] ggplot2_0.8.8 proto_0.3-8 reshape_0.8.3 plyr_1.0.3

loaded via a namespace (and not attached):

[1] digest_0.4.2 tools_2.11.1

My mistake - I see the problem. My "extreme" example wasn't different
at all. Try changing 0.05 to 0.5. You should see something like the
plot I've attached.

Bob

[baptiste auguie]

Hi,

Can you not use expand_limits() for this?

p + geom_point() + facet_wrap(~
RandFactor, ncol=1, scales="free_y") +
scale_y_continuous(expand=c(0,0)) + expand_limits(y=c(-20,20))

HTH,

baptiste

[Bob Chan]

Thanks for the suggestion, Baptiste. Unfortunately, my example was not
complex enough to show why I wanted to use expand(). In my dataset,
the two facets have slightly different ranges, so a fixed ylim won't
work. If I can modify my example as follows:

economics$RandFactor <- c("FactorA","FactorB")

economics$psavert[economics$RandFactor == "FactorB"] <-
economics$psavert[economics$RandFactor == "FactorB"] - 30

p <- ggplot(economics, aes(x=date, y=psavert))

p + geom_point() + facet_wrap(~ RandFactor, ncol=1, scales="free_y")

Now you can see that static expanded limits won't work. I want the
data in each facet to be "centered" vertically, but to expand the
y-limits for each facet so that the minor fluctuations in each facet
are visually minimized.

[Hadley Wickham]

Hi Bob,

You're using the expand parameter for a purpose it wasn't designed for
- the point of it is to give a little space between the data and the
ticks, so adding ticks in that gap would defeat the purpose. However,
if you look at the source code for expand_limits, you can see
modifying for your needs is pretty simple. You just construct a data
frame that provides both the y values and the facet values and then
use that with geom_blank.

ranges <- data.frame(RandFactor = c("a", "a", "b", "b"), y = c(1, 100,
1000, 10000))
p + geom_blank(aes(y = y), data = ranges, inherit.aes = FALSE)

Hadley

--
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/