How to get a vertical line in a scatter polar coordinates plot in R using ggplot2

I have two questions:

1. Hopefully a simple question, but I want to put a vertical line in my polar coordinates graph that denotes the start of the data. I have a data set that collects hourly readings over the period of a year and I have managed (with help) to get the polar graph I wanted. My data is in the format (x = yyyy-mm-dd HH:MM:SS and y = number value). I have rotated the origin so that the start is not vertical (I need to keep this rotation). So to denote where the data starts and ends over a period of a year, I would like to add in a vertical line (see image below).

2. If this is not possible, is it possible to put in an empty space between the start and end dates, to leave a gap in the data, as a visual representation of the start and end time?

Here is my current example dataset and code:

library(lubridate) #make an example dataset
NoOfHours <- as.numeric(ymd_hms("2019-6-1 17:00:00") - ymd_hms("2018-3-01 8:00:00"))*24 
data <- as.data.frame(ymd_hms("2018-3-01 8:00:00") + hours(0:NoOfHours))
colnames(data) <- 'date' 
set.seed(10)
data$level <- runif(nrow(data), min = 0, max = 250)

library(openair)
yeardata <- selectByDate(data, start = "2018-3-1", end = "2019-2-28", year = 2018:2019)

library(ggplot2)

plot <- ggplot(yeardata, aes(x=date, y=level, color = level)) +
  geom_hline(yintercept = seq(0, 300, by = 50), colour = "black", size = 0.75, alpha = 0.3)+ #make my own gridlines so that when on a white background, the gridlines wont cross the text.
  geom_vline(xintercept = as.POSIXct(data$date[1], origin = "1970-01-01 00:00:00 UTC"), colour = "black", size = 0.75, alpha = 0.3, )+
  scale_color_gradient(limits = c(0,200), low="green", high="red", oob = scales::squish)+ #need oob = scales::squish to get values over 200 to be red.
    geom_jitter(alpha = 0.2, size = 2) +# Use a slightly darker palette than normal
 theme(axis.title=element_text(size=16,face="bold"), axis.text.x = element_text(size = 16), axis.text.y = element_text(size = 12))+
   labs(x = NULL, y = bquote('Levels '~(m^2)), color = "Level")+ #bquote to allow superscripts
  scale_y_continuous(breaks = seq(0, 300, 50),
                     limits = c(-100,310))
plot
plot + coord_polar(start = ((2*60/365)*pi))+ #need to have the number of radians to get my start position. If march 1st is the start date, then 60 days have past since Jan 1.
  theme(legend.title = element_text(color = "black", size = 14, face = "bold"), panel.background = element_rect(fill = "white"), panel.grid  = element_blank())

I can get a normal scatter plot, with the vertical line where I want it (at the start of the dataset) that looks like this:

but when I try to put this into polar coordinates I get this error:

"Error in as.POSIXct.numeric(value) : 'origin' must be supplied"

I have attempted to supply the origin in a few formats with the lastest being what is seen in the code above. Ideally, the graph would look something like this:

Any help would be appreciated.

Thanks

Here's an approach to add the line and text labels. It looks like an unintended consequence of adding the first text annotation, with x prior to the first date, is creating a gap like you requested in #2. But a downside is that it slightly changes the rotation of the breaks, which might require more adjustment of the coord_polar(start = term.

 ggplot(yeardata, aes(x=date, y=level, color = level)) +
   geom_hline(yintercept = seq(0, 300, by = 50), colour = "black", size = 0.75, alpha = 0.3)+ 
   # this annotation layer is drawn over the gridlines, but under the points
   annotate("segment",x = ymd_hms(data$date[1]), xend = ymd_hms(data$date[1]),
                      y = 0, yend = 310, colour = "black", size = 0.75, alpha = 0.3) +


  scale_color_gradient(limits = c(0,200), low="green", high="red", oob = scales::squish)+       
  geom_jitter(alpha = 0.2, size = 2) +# Use a slightly darker palette than normal

  # this text is prior to existing x data and creates a gap
  annotate("text", label = "my text", x = ymd_hms(data$date[1]) - ddays(5), 
            y = 300, angle = 0) +
  annotate("text", label = "my text2", x = ymd_hms(data$date[1]) + ddays(5), 
            y = 300, angle = 0) +
  theme(axis.title=element_text(size=16,face="bold"), axis.text.x = element_text(size = 16), axis.text.y = element_text(size = 12))+
  labs(x = NULL, y = bquote('Levels '~(m^2)), color = "Level")+ #bquote to allow superscripts
  scale_y_continuous(breaks = seq(0, 300, 50),
                     limits = c(-100,310))+
  coord_polar(start = ((2*60/365)*pi))+      
  theme(legend.title = element_text(color = "black", size = 14, face = "bold"),   
        panel.background = element_rect(fill = "white"), 
        panel.grid  = element_blank())