Changing x-axis to discrete alters margins - r

Question

I have a plot produced with the following code:
plot <- ggplot(lmeans, aes(x=Day, y=value*100, group=variable, colour=variable)) +
geom_point(aes(shape=variable), size=4) +
geom_line(aes(linetype=variable), size=1.5) +
ggtitle(paste("Nausea and Vomitting Frequencies by Day for", group_name)) +
ylab("Frequency (%)") +
ylim(0, 40) +
theme(legend.title=element_blank()) +
theme(legend.justification = c(1, 1), legend.position = c(1, 1))
Which results in a plot like so:
However I would like the days to be discretely labeled rather than being given as a continuous axis. When I try to achieve this by adding scale_x_discrete(), I get the following result:
In which the 'margins' on the x-axis are altered in an unsightful manner. How can I avoid these unsightly changes?
Here's a minimal example for reproduction:
require(ggplot2)
lmeans <- data.frame(Day=c(0,1,2,3,0,1,2,3),
variable=c("x","x","x","x","y","y","y","y"),
value=c(5,4,2,1,7,3,2,0))
plot <- ggplot(lmeans, aes(x=Day, y=value, group=variable, colour=variable)) +
geom_point(aes(shape=variable)) +
geom_line(aes(linetype=variable)) +
ylim(0, 10) +
scale_x_discrete() +
theme(legend.justification = c(1, 1), legend.position = c(1, 1))
print(plot)
Which produces this:

Answer 1

Without scale_x_discrete and setting Day as a factor, the plot is looking OK:
ggplot(lmeans, aes(x=factor(Day), y=value, group=variable, colour=variable)) +
geom_point(aes(shape=variable), size=4) +
geom_line(aes(linetype=variable), size=1.5) +
theme(legend.justification = c(1, 1), legend.position = c(1, 1))
which gives:
When you use scale_x_discrete, you can include the expand parameter in order to set the margins. An example:
ggplot(lmeans, aes(x=factor(Day), y=value, group=variable, colour=variable)) +
geom_point(aes(shape=variable), size=4) +
geom_line(aes(linetype=variable), size=1.5) +
ylim(0, 10) +
scale_x_discrete("Day", expand=c(0.05,0.1), breaks=c(0,1,2,3)) +
theme(legend.justification = c(1, 1), legend.position = c(1, 1))
which gives:

Answer 2

I changed scale_x_discrete() to scale_x_continuous() and used limits. Does this work for you?
ggplot(lmeans, aes(x=Day, y=value, group=variable, colour=variable)) +
geom_point(aes(shape=variable)) +
geom_line(aes(linetype=variable)) +
ylim(0, 10) +
scale_x_continuous(limits = c(-0.5, 4)) +
theme(legend.justification = c(1, 1), legend.position = c(1, 1))

Answer 3

Use factor(Day) in the aesthetic mapping:
plot <- ggplot(lmeans, aes(x=factor(Day), y=value,
group=variable, colour=variable)) +
geom_point(aes(shape=variable)) +
geom_line(aes(linetype=variable)) +
ylim(0, 10) +
labs(x="Day") +
theme(legend.justification=c(1, 1), legend.position=c(1, 1))
print(plot)