Invalid trim argument in plotting counts over dates in R - r

Question

I am trying to apply the answer to my prior question on plotting with dates in the x axis to the COVID data in the New York Times but I get an error message:
require(RCurl)
require(foreign)
require(tidyverse)
counties = read.csv("https://raw.githubusercontent.com/nytimes/covid-19-data/master/us-counties.csv", sep =",",header = T)
Philadelphia <- counties[counties$county=="Philadelphia",]
Philadelphia <- droplevels(Philadelphia)
rownames(Philadelphia) <- NULL
with(as.data.frame(Philadelphia),plot(date,cases,xaxt="n"))
axis.POSIXct(1,at=Philadelphia$date,
labels=format(Philadelphia$date,"%y-%m-%d"),
las=2, cex.axis=0.8)
# Error in format.default(structure(as.character(x), names = names(x), dim = dim(x), :
# invalid 'trim' argument
The structure of the data includes already a date format:
> str(Philadelphia)
'data.frame': 21 obs. of 6 variables:
$ date : Factor w/ 21 levels "2020-03-10","2020-03-11",..: 1 2 3 4 5 6 7 8 9 10 ...
$ county: Factor w/ 1 level "Philadelphia": 1 1 1 1 1 1 1 1 1 1 ...
$ state : Factor w/ 1 level "Pennsylvania": 1 1 1 1 1 1 1 1 1 1 ...
$ fips : int 42101 42101 42101 42101 42101 42101 42101 42101 42101 42101 ...
$ cases : int 1 1 1 3 4 8 8 10 17 33 ...
$ deaths: int 0 0 0 0 0 0 0 0 0 0 ...
I tried changing the axis call to
axis.Date(1,Philadelphia$date, at=Philadelphia$date,
labels=format(Philadelphia$date,"%y-%m-%d"),
las=2, cex.axis=0.8)
without success.
I wonder if it has to do with the strange horizontal lines in the plot (as opposed to points):

Answer 1

The 'invalid trim argument' error comes from format (it is the default second argument because you haven't explicitly specified the parameter).
I'm not entirely sure what you're doing here but I would change date to a Date object before plotting the data. You'll also want to use %Y instead of %y I believe.
library(dplyr)
counties = read.csv("https://raw.githubusercontent.com/nytimes/covid-19-data/master/us-counties.csv", sep =",",header = T)
Philadelphia <- counties[counties$county=="Philadelphia",] %>%
mutate(date = as.POSIXct(date, format = '%Y-%m-%d'))
with(Philadelphia, plot(date,cases))