ggplot overlay plot error, no layers in plot

Question

I am trying to overlay two variables on a same figure with ggplot2, so I use melt to get the data in the correct format and then use the following:

Locations <- c("USA","UK","Spain")
vals_1 <- c(44,6,76)
vals_2 <- c(0.2,0.9,4.1)

dat <- data.frame(Locs = Locations,
                  method_1 = vals_1,
                  method_2 = vals_2)
dat2 <- melt(dat,id = "Locs")

ggplot(data = dat2,
       aes(x = Locs, y = value, colour = variable))

but this generates an error. Why does it state that there are no layers?

Is this to do with the class of dat2[,1] and dat2[,2] being a factor? If so, what should it be changed to? I would like the graph to show the string in dat2[,1] on the xaxis and both the variables shown in the plot. Could someone point me in the right direction?

Amend:

After adding

geom_line()

to get

ggplot(data = dat2,
       aes(x = Locs, y = value, colour = variable)) +
  geom_line()

I receive the following error geom_path: Each group consist of only one observation. Do you need to adjust the group aesthetic?

You haven't added any geom_ to your plot, for example, +geom_bar() — Didzis Elferts
– Didzis Elferts, Commented Aug 16, 2013 at 8:01

Didzis Elferts · Accepted Answer · 2013-08-16 08:11:43Z

2

You have to tell ggplot() which points to connect by lines. This is done by adding group=variable inside the aes().

ggplot(data = dat2,
       aes(x = Locs, y = value, colour = variable,group=variable)) +
  geom_line()

answered Aug 16, 2013 at 8:11

Didzis Elferts

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