```{r question 3, fig.cap=" ", echo = FALSE, message = FALSE, warning=FALSE}
covid19$new_date <- paste(substr(covid19$created_at,1,10), substr(covid19$created_at,12,16),sep = " ")
colnames(covid19)

covid19timetrend <- covid19%>%
  mutate(freq = 1)%>%
  dplyr::select(new_date,freq)%>%
  group_by(new_date)%>%
  summarise(no_tweets = sum(freq))

head(covid19timetrend)

covid19timetrend_figure <- ggplot(covid19timetrend,aes(x=new_date,y=no_tweets))+
  geom_line(aes(y=no_tweets),colour="blue")


print(covid19timetrend_figure)



我这个线图就是不出线。。怎么办，求助各位大佬谢谢！1