apepper - 1 year ago 114
R Question

# Context

I want to plot two ggplot2 on the same page with the same legend. http://code.google.com/p/gridextra/wiki/arrangeGrob discribes, how to do this. This already looks good. But... In my example I have two plots with the same x-axis and different y-axis. When the range of the the y-axis is at least 10 times higher than of the other plot (e.g. 10000 instead of 1000), ggplot2 (or grid?) does not align the plots correct (see Output below).

# Question

How do I also align the left side of the plot, using two different y-axis?

# Example Code

x = c(1, 2)
y = c(10, 1000)
data1 = data.frame(x,y)
p1 <- ggplot(data1) + aes(x=x, y=y, colour=x) + geom_line()

y = c(10, 10000)
data2 = data.frame(x,y)
p2 <- ggplot(data2) + aes(x=x, y=y, colour=x) + geom_line()

leg <- ggplotGrob(p1 + opts(keep="legend_box"))
legend=gTree(children=gList(leg), cl="legendGrob")
widthDetails.legendGrob <- function(x) unit(3, "cm")
grid.arrange(
p1 + opts(legend.position="none"),
p2 + opts(legend.position="none"),
legend=legend, main ="", left = "")

# Output

If you don't mind a shameless kludge, just add an extra character to the longest label in p1, like this:

p1 <- ggplot(data1) +
aes(x=x, y=y, colour=x) +
geom_line() +
scale_y_continuous(breaks = seq(200, 1000, 200),
labels = c(seq(200, 800, 200), " 1000"))

I have two underlying questions, which I hope you'll forgive if you have your reasons:

1) Why not use the same y axis on both? I feel like that's a more straight-forward approach, and easily achieved in your above example by adding scale_y_continuous(limits = c(0, 10000)) to p1.

2) Is the functionality provided by facet_wrap not adequate here? It's hard to know what your data structure is actually like, but here's a toy example of how I'd do this:

library(ggplot2)

# Maybe your dataset is like this
x <- data.frame(x = c(1, 2),
y1 = c(0, 1000),
y2 = c(0, 10000))

# Molten data makes a lot of things easier in ggplot
x.melt <- melt(x, id.var = "x", measure.var = c("y1", "y2"))

# Plot it - one page, two facets, identical axes (though you could change them),
# one legend
ggplot(x.melt, aes(x = x, y = value, color = x)) +
geom_line() +
facet_wrap( ~ variable, nrow = 2)
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