When I loaded the data "tradingDay" had a character class, so R was treating it like text, not a date.
I think that may have been your issue.
I'm not sure what you're going for in the end, but maybe something like this?
library(tidyverse)
library(lubridate)
# Coerce tradingDay to date format (and make data a tibble, unnecessary, but nice)
data <- as_tibble(data) %>%
mutate(tradingDay = as_date(tradingDay))
# Simple plot of time series
ggplot(data, aes(x = tradingDay, y = close)) + geom_line()
I hope that helps,
Luke