Error plotting with ggplot

Question

library(tidyverse)
library(lubridate)

Suppose a dataset containing the year followed by the number of books published by an author:

df<-tibble(
  ano = c(2007, 2008, 2017, 2018),
  n = c(1, 2, 2, 1)
)

df

# A tibble: 4 x 2
    ano     n
  <dbl> <dbl>
1  2007     1
2  2008     2
3  2017     2
4  2018     1

If I try to create a chart with:

df %>% 
  ggplot(aes(ano, n))+
  geom_line()

...I get the following:

My interpretation of the graph generated (correct me if I’m wrong, please), is that, according to the graph, 1 book was published in 2007, 2 books in 2008, and 2 books each year between 2008 and 2017 and, again, 1 book in 2018.

The issue is that, in fact, between 2008 and 2017 there is no record.

I imagined that by having put the year simply as a number (that is, without having classified it as a year), it had caused this misunderstanding.

So, I converted the "year" column into years (in fact) with the expectation of solving the question. I used the function ymd() of lubridate and saved in a new dataset called "df2":

df2<-df %>% 
  mutate(ano = ymd(paste0(ano, "01", "01")))

df2

# A tibble: 4 x 2
  ano            n
  <date>     <dbl>
1 2007-01-01     1
2 2008-01-01     2
3 2017-01-01     2
4 2018-01-01     1

And plotted the graph:

df2 %>% 
  ggplot(aes(ano, n))+
  geom_line()+
  theme_bw()

However, as a result, I had exactly the same chart as before:

... I don’t know, my expectation is that the correct graph would be a line (going up) that goes from x=2007, y=1 until x=2008, y=2 -- then another line (going down) that goes from x=2017, y=2 until x=2018, y=1. In other words, in my opinion there should not be this line (straight) linking 2008 to 2018 at y=2.

Where is my mistake?

Answer 1

An example creating the sequence of dates with n equal to zero

library(ggplot2)
library(dplyr)

df <-data.frame(
  ano = c(2007, 2008, 2017, 2018),
  n = c(1, 2, 2, 1)
)

anos <- data.frame(ano = 2007:2018)

left_join(anos, df, by = 'ano') %>%  replace(is.na(.), 0) %>%
  ggplot( aes(factor(ano),factor(n), group = 1) )+
    geom_line()

Answer 2

One solution is to use the function tidyr::complete.

df %>% 
  complete(ano = 2007:2018, fill = list(n = 0)) %>%
  ggplot(aes(ano, n))+
  geom_line()