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I am a newie here, sorry for not writing the question right :p

1, the aim is to plot a graph about the mean NDVI value during a time period (8 dates were chosen from 2019-05 to 2019-10) of my study site (named RB1). And plot vertical lines to show the date with a grass cutting event.

2, Now I had calculated the NDVI value for these 8 chosen dates and made a CSV file. (PS. the "cutting" means when the grassland on the study site has been cut, so the corresponding dates should be show as a vertical line, using geom_vline)

infor <- read_csv("plotting information.csv")
infor
# A tibble: 142 x 3
   date         NDVI cutting
   <date>      <dbl> <lgl>  
 1 2019-05-12 NA     NA     
 2 2019-05-13 NA     NA     
 3 2019-05-14 NA     NA     
 4 2019-05-15 NA     NA     
 5 2019-05-16 NA     NA     
 6 2019-05-17  0.787 TRUE      
# ... with 132 more rows

3, the problem is, when I do the ggplot, first I want to keep the x-axis as the whole time period (2019-05 to 2019-10) but of course not show all dates in between, otherwise there will be way too much dates show on the x-axis). So, I do the scale_x_discrte(breaks=, labels=) to show the specific dates with NDVI values.

Second I also want to show the dates that the grasses were cut geom_vline.

BUT, it seems like the precondition for scale_x_discrte is to factor my date, while the precondition for geom_vline is to keep the date as nummeric. these two calls seems to be contradictory. 

y1 <- ggplot(infor, aes(factor(date), NDVI, group = 1)) +
  geom_point() +
  geom_line(data=infor[!is.na(infor$NDVI),]) + 
  scale_x_discrete(breaks = c("2019-05-17", "2019-06-18", "2019-06-26", "2019-06-28","2019-07-23","2019-07-28", "2019-08-27","2019-08-30", "2019-09-21"), 
                   labels = c("0517","0618","0626","0628","0723","0728", "0827","0830","0921"))) 


y2 <- ggplot(infor, aes(date, NDVI, group = 1)) +
  geom_point() +
  geom_line(data=infor[!is.na(infor$NDVI),]))

when I add the geom_vline in the y1, vertical lines do not show on my plot: y1 + geom_vline

when I add it in the y2, vertical lines were showed, but the dates (x axis) are weird (not show as the y1 because we donot run the scale_x_ here) y2 + geom_vline

   y1 + 
      geom_vline(data=filter(infor,cutting == "TRUE"), aes(xintercept = as.numeric(date)), color = "red", linetype ="dashed")

Would be appreciated if you can help! thanks in advance! :D

Chryseis Ho
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  • It's easier to help you if you include a simple [reproducible example](https://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example) with sample input and desired output that can be used to test and verify possible solutions. – MrFlick Nov 19 '19 at 16:21
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    You've turned the date variable on your x-axis into a factor in your `ggplot` call, but then try to set the x-intercept by making the dates numeric. There's likely a conflict between those two, since now they'll have different ranges. Honestly, neither seems like a good approach—why not keep dates as dates? – camille Nov 19 '19 at 17:25
  • thanks for your advice! I turned the date into a factor because this can make sure the scale_x_discrete works. And why I need to do this scale_x is because I want to keep all the time period (2019-05 to 2019-10) on my x-axis, but at the same time just show the specific dates with a NDVI. So I do the breaks and labels. – Chryseis Ho Nov 20 '19 at 10:18
  • I have edited my question with some graphs. Hope I describe my question more clear. You could also have a look ;) – Chryseis Ho Nov 20 '19 at 10:50

1 Answers1

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I agree with the comment about leaving dates as dates. In this case, you can specify the x-intercept of geom_vline as a date.

Given basic data:

df <- tribble(
  ~Date, ~Volume, ~Cut,
  '1-1-2010', 123456, 'FALSE',
  '5-1-2010', 789012, 'TRUE',
  '9-1-2010', 5858585, 'TRUE',
  '12-31-2010', 2543425, 'FALSE'
)

I set the date and then pull the subset for Cut=='TRUE' into a new object:

df <- mutate(df, Date = lubridate::mdy(Date))

d2 <- filter(df, Cut == 'TRUE') %>% pull(Date)

And finally use the object to specify intercepts:

df %>%
  ggplot(aes(x = Date, y = Volume)) +
  geom_vline(xintercept = d2) +
  geom_line()
AHart
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  • thanks for the answers! :) I run your code and the result is like what I got in my y2 + geom_vline. The same problem is that the x-axis doesn't show the exact dates (only show as Jan 2010, Apr 2010....). And that's the reason why I try to apply scale_x_discrete to break and label the specific dates (2010-05-01, 2019-09-01 for example). – Chryseis Ho Nov 20 '19 at 10:26
  • I have edited my question with some graphs. You could also have a look ;) – Chryseis Ho Nov 20 '19 at 10:49