How to add a label to horizontal line in ggplot 2 when x-axis is date?

I am trying to graph a temperature dataset using mean, max, and min temps by month over 2 years. The graph includes two horizontal temperature thresholds.

I have succeeded in creating a graph, but I want to add labels "9.9" and "12.97" to my 2 horizontal threshold lines, and am having trouble I think because the x-axis is a date.

Here is the dput() sample of my data (hob_m_cs1_sort):

structure(list(year = c(2021, 2021, 2021, 2021), month = c(2, 
3, 4, 5), tmin_mean = c(10.625, 8.27870967741936, 7.78666666666667, 
9.34225806451613), tmax_mean = c(15.255, 15.8003225806452, 16.869, 
18.6835483870968), tmean = c(12.3655534638554, 11.5371012544803, 
11.9291921296296, 13.5006406810036), date = structure(c(18659, 
18687, 18718, 18748), class = "Date"), month_name = c("Feb", 
"Mar", "Apr", "May")), row.names = c(NA, 4L), class = "data.frame")`

This is the code I have been using:

hob_m_cs1_sort %>% group_by(date) %>%
  summarise(min = min(tmin_mean, na.rm = TRUE),
            max = max(tmax_mean, na.rm = TRUE),
            avg = mean(tmean,na.rm = TRUE)) %>%
  gather(metric, value, -date) %>%
  ggplot(.,aes(x = date, y = value, 
               group = metric, color = metric)) + 
  labs(color='Temperature') +
  ggtitle ("Hakalau Monthly Temperatures: Pua 'Akala, 1510 m") +
  theme(plot.title = element_text(hjust = 0.5)) +
  xlab("Date") +  ylab ("Temperature ( ºC )") +
  scale_y_continuous(limits = c(2.5, 22.5), breaks = seq(5, 25, by = 5)) +
  scale_x_date(date_breaks = "2 months", date_labels = "%b %Y") +
  theme_ipsum() +
  theme(axis.text.x=element_text(angle=60, hjust=1)) +
  geom_line(aes(color = metric)) + 
  geom_hline(aes(yintercept=h, linetype = "Culex development"), colour= 'darkorange1') +
  geom_hline(aes(yintercept=h2, linetype = "Avian malaria development"), colour= 'red') +
  scale_linetype_manual(name = "Temperature Thresholds", values = c(2, 2), 
                        guide = guide_legend(override.aes = list(color = c("red", "darkorange1")))) +
  scale_color_manual(values = c("steelblue1", "navyblue", "darkturquoise"), breaks=c('max', 'avg', 'min'), labels=c('Max', 'Avg', 'Min'))

I am able to produce this graph, but no labels on the thresholds: link below

I have tried these options but they are not producing labels for me:

geom_text(aes(0, h, label = h, vjust = - 1)) +
  geom_text(aes(0, h2, label = h2, vjust = - 1)) +

geom_text(aes("2021-02-01", h, label = h)) +
  geom_text(aes("2021-02-01", h2, label = h2)) +
  
annotate(y= 9.9, x = dmy("01/02/2021"), label="Normal Limit", geom = "label")

Please help! Thanks :)

Solution

You need to remind R that you're dealing with dates. You can use lubridate::as_date. I've removed a good deal of code that wasn't necessary for the problem.

May I suggest using vectors for annotation instead, thus you will need only one call to annotate.
May I suggest the geomtextpath package and direct labelling of your lines with a proper label and not the value. Why? The value is already represented by the very height of the line. And the direct label will make it easier for the reader to understand the meaning of the line.

Smaller comments / suggestions in the code

library(tidyverse)
library(lubridate)
#> Loading required package: timechange
#> 
#> Attaching package: 'lubridate'
#> The following objects are masked from 'package:base':
#> 
#>     date, intersect, setdiff, union
library(geomtextpath)
hob_m_cs1_sort <- structure(list(year = c(2021, 2021, 2021, 2021), month = c(2, 
                                                           3, 4, 5), tmin_mean = c(10.625, 8.27870967741936, 7.78666666666667, 
                                                                                   9.34225806451613), tmax_mean = c(15.255, 15.8003225806452, 16.869, 
                                                                                                                    18.6835483870968), tmean = c(12.3655534638554, 11.5371012544803, 
                                                                                                                                                 11.9291921296296, 13.5006406810036), date = structure(c(18659, 
                                                                                                                                                                                                         18687, 18718, 18748), class = "Date"), month_name = c("Feb", 
                                                                                                                                                                                                                                                               "Mar", "Apr", "May")), row.names = c(NA, 4L), class = "data.frame")


h <- 9.9
h2 <- 12.97

## I like to store as a proper data frame if more than one manipulation step 
hob_long <- hob_m_cs1_sort %>% group_by(date) %>%
  summarise(min = min(tmin_mean, na.rm = TRUE),
            max = max(tmax_mean, na.rm = TRUE),
            avg = mean(tmean,na.rm = TRUE)) %>%
  gather(metric, value, -date)

ggplot(hob_long, aes(x = date, y = value, group = metric, color = metric)) + 
  ## removed aes, as specified in main ggplot call 
  geom_line() + 
  geom_hline(aes(yintercept=h, linetype = "Culex development"), colour= 'darkorange1') +
  geom_hline(aes(yintercept=h2, linetype = "Avian malaria development"), colour= 'red') +
  ## do both in one call, use vectors
  annotate("text", x = as_date(c("2021-02-01", "2021-02-01")), y = c(h, h2), label = c(h, h2))


## how I would do the plot
ggplot(hob_long, aes(x = date, y = value, group = metric, color = metric)) + 
  geom_line() + 
  geom_texthline(aes(yintercept=h, label = "Culex development"), lty = 2, colour= 'darkorange1') +
  geom_texthline(aes(yintercept=h2, label = "Avian malaria development"), lty = 2, colour= 'red')

^{Created on 2023-01-18 with reprex v2.0.2}