R adding dynamic reference line to ggplot

Question

I tried to use the solution mentioned in this Add vline to existing plot and have it appear in ggplot2 legend?

However, part of the code doesn't work for my case especially the vline part didn't work. I'm wondering if there are others trick? I have two data set first one is the main dataset:

str(sf)
$ date               : chr  "2020-02-02" "2020-02-03" "2020-02-04" "2020-02-05" ...
$ School_Closure_Date: chr  "2020-03-16" "2020-03-16" "2020-03-16" "2020-03-16" ...
$ variable           : Factor w/ 5 levels "cases","deaths",..: 1 1 1 1 1 1 1 1 1 1 ...
$ value              : num  2 2 2 2 2 2 2 2 2 2 ...
$ mindate            : Date, format: "2020-01-24" "2020-01-24" …
$ maxdate            : Date, format: "2020-03-30" "2020-03-30" ...
$ closure            : logi  FALSE FALSE FALSE FALSE FALSE FALSE …

 str(sfs)
'data.frame':   1 obs. of  4 variables:
 $ fips: int 6075
 $ City: chr "San_Francisco"
 $ date: chr "2020-03-16"
 $ type: chr "school closure"

My pupose is to put a vline to the ggplot. However, I got error messeges:

        y <- ggplot(sf,aes(date,value)) +            geom_line(aes(group=variable,color=variable))
        x <- geom_vline(data=sfs,xintercept=date,show.legend=TRUE)
        y+x

        Error in UseMethod("rescale") : 
        no applicable method for 'rescale' applied to an object of      class "factor"
        In addition: Warning message:
        Removed 33 rows containing missing values (geom_path). 

The y part works but after adding part x, the figure disappeared. Any thought? Thanks for your help!