[R] legend order in ggplot2

[Tom Hopper]

John,

The order of legends in ggplot2 depends on the order of factor levels in the data frame. The linetype can be matched to the factor levels using a named vector (ggplot2 basically does a lookup).

The biggest problem you have here is that you’re not passing data in the right form or format to ggplot2. ggplot2 expects a data frame in tidy format (see http://r4ds.had.co.nz/tidy-data.html), and you’ve got wide (and pretty messy) data.

To get this to work, your data frame needs to look like:

x    var           val
1    name_a   1
2    name_a   2
1    name_b   3
2    name_b   4
.
.
.

To make this work, I’m going to load tidyverse instead of just ggplot2. We’ll start with your data, but we need to reshape it into tidy format using dplyr and tidyr, and then we need to order the factor levels with forcats.

library(tidyverse)
my_df <- data_frame(x = c(1, 2),
               name_a=c(1,2),
               name_b=c(3,4),
               name_c=c(5,6)) %>%
  gather(var, val, -x) %>%
  mutate(var = fct_relevel(var, "name_b", "name_a", “name_c”))

Here’s how the data looks:

> my_df
# A tibble: 6 x 3
      x var      val
  <dbl> <fct>  <dbl>
1     1 name_a     1
2     2 name_a     2
3     1 name_b     3
4     2 name_b     4
5     1 name_c     5
6     2 name_c     6
> levels(my_df$var)
[1] "name_b" "name_a" “name_c"

Now we have the data in the right format, and your factors are in the right order for the legend. We can quick test this:

ggplot(my_df, aes(x = x, y = val, linetype = var)) +
  geom_line()

and I think this is pretty close to what you were looking for.

To control the linetype by factor level, we need to tell ggplot2 which linetype goes with which factor level by using scale_linetype_manual():

my_lines <- c(name_a = "solid", name_b = "dotted", name_c = "twodash")

ggplot(df, aes(x = x, y = val, linetype = var)) +
  geom_line() +
  scale_linetype_manual(values = my_lines)

Here, I’ve deliberately changed the linetypes so we can see that the code works as desired. You’ll want to change the linetypes in my_lines to what you want.

Putting it all together:

library(tidyverse)
my_df <- data_frame(x = c(1, 2),
               name_a=c(1,2),
               name_b=c(3,4),
               name_c=c(5,6)) %>%
  gather(var, val, -x) %>%
  mutate(var = fct_relevel(var, "name_b", "name_a", "name_c”))

my_lines <- c(name_a = "solid", name_b = "dotted", name_c = "twodash")

ggplot(my_df, aes(x = x, y = val, linetype = var)) +
  geom_line() +
  scale_linetype_manual(values = my_lines)

Regards,

Tom



> On 201805 21, at 23:16, John <miaojpm using gmail.com> wrote:
> 
> Hi,
> 
>   I'd like to graph three lines on ggplot2 and I intend the lines to be
> "solid", "dashed", and "dotted". The legend names are "name_b", "name_a",
> "name_c". I'd like to legend to present in the order: the "name_b" at the
> top, and "name_c" at the bottom.
> As a consequence, the legend is indeed in the order: name_b at the top and
> name_c at the bottom. However, I'd like name_b to corresponds to "solid",
> while it corresponds to "dashed", etc, which I don't want. How could I make
> "solid" correspond to "name_b"? Thanks,
> 
> #########
> 
> library(ggplot2)
> df<-data.frame(x1=c(1,2), y1=c(3,4),z1=c(5,6),w1=c(7,8))
> p1<-ggplot(df, aes(x=1:2, y=x1))+
>  geom_line(aes(linetype="name_b"))+
>  geom_line(aes(x=1:2, y=y1, linetype="name_a"), df)+
>  geom_line(aes(x=1:2, y=z1, linetype="name_c"), df)+
>  scale_linetype_manual(name="", values=c("solid","dashed", "dotted"),
> breaks=c("name_b","name_a","name_c"))
> ###########
> 
> 	[[alternative HTML version deleted]]
> 
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