ggplot - Set colour of lines depending on a variable with a changing presence of the variable type within the data

Question

Assume the following data frame:

mydf <- data.frame(date = as.Date(rep(c('2019-11-01', '2019-10-01'), 2)), 
                    value = c(10, 15, 8, 4),
                    type = c('Type 1', 'Type 1', 'Type 2', 'Type 2'))

print(mydf)
        date value   type
1 2019-11-01    10 Type 1
2 2019-10-01    15 Type 1
3 2019-11-01     8 Type 2
4 2019-10-01     4 Type 2

I want to create an automated code which creates a line plot for each type and define the colours of each line. Generally, I know how to do that:

require(ggplot2)
myplot <- ggplot(mydf, aes(x = date, y = value, colour = type)) + geom_line() +
  scale_color_manual(name = 'Type', values=c('blue', 'red'))

However, the data frame might be changing when running the code in another month. There might be a Type 3 within the data frame:

mydf <- data.frame(date = as.Date(rep(c('2019-11-01', '2019-10-01'), 3)), 
                    value = c(10, 15, 8, 4, 12, 8),
                    type = c('Type 1', 'Type 1', 'Type 2', 'Type 2', 'Type 3', 'Type 3'))

print(mydf)
     date    value  type
1 2019-11-01    10 Type 1
2 2019-10-01    15 Type 1
3 2019-11-01     8 Type 2
4 2019-10-01     4 Type 2
5 2019-11-01    12 Type 3
6 2019-10-01     8 Type 3

And in yet another month Type 1 or Type 2 might not be in the data:

mydf <- data.frame(date = as.Date(rep(c('2019-11-01', '2019-10-01'), 2)), 
                    value = c(10, 15, 8, 4),
                    type = c('Type 1', 'Type 1', 'Type 3', 'Type 3'))

print(mydf)
        date value   type
1 2019-11-01    10 Type 1
2 2019-10-01    15 Type 1
3 2019-11-01     8 Type 3
4 2019-10-01     4 Type 3

How can I set the colours for Type 1, Type 2 and Type 3 and then variably use the respective defined colours depending on which Type is present in the data. So I can pre-define the colours and just run the script with the new data without needing to change anything within my code (assume Type 1 should be blue, Type 2 should be red and Type 3 should be black for each plot of the three data frames). Thanks!

Answer 1

The values parameter can take a named vector to assign values to respective Type.

library(ggplot2)

cols <- c('Type 1' = 'blue', 'Type 2' = 'red', 'Type 3' = 'black')

ggplot(mydf, aes(x = date, y = value, colour = type)) + geom_line() +
  scale_color_manual(name = 'Type',values= cols)

so when you have data with all types present, it looks

mydf <- data.frame(date = as.Date(rep(c('2019-11-01', '2019-10-01'), 3)), 
             value = c(10, 15, 8, 4, 12, 8),
             type = c('Type 1', 'Type 1', 'Type 2', 'Type 2', 'Type 3', 'Type 3'))

and when you have some types absent, it still uses the same colors with same code.

mydf <- data.frame(date = as.Date(rep(c('2019-11-01', '2019-10-01'), 2)), 
                value = c(10, 15, 8, 4),
                type = c('Type 1', 'Type 1', 'Type 3', 'Type 3'))