library(ggplot2)

N <- 30

Date <- economics$date[1:N]
Pop <- economics$pop[1:N]

DateCount<-c(1:N)

df <- data.frame(DateCount,Date,Pop)

trendCoef <- lm(Pop ~ DateCount)
k<-trendCoef$coefficients[[2]]
b<-trendCoef$coefficients[[1]]
x <- c(1:N)
y <- c()
for(i in x){
  y <- append(y, k*i+b)
}
trend <- data.frame(x,y)

ggplot()+geom_line(data=trend, aes(x, y), color="red")+geom_line(data=df, aes(DateCount, Pop))