Draw trend lines on a scatterplot with datetime variable - r

Question

I have a data frame of almost 1600 observations with this structure:
head(df)
Start_Time Duration
1 2014-09-18 10:01:00 4 mins
2 2014-09-18 08:01:00 41 mins
3 2014-09-18 08:01:00 22 mins
4 2014-09-18 08:01:00 41 mins
5 2014-09-18 08:01:00 60 mins
6 2014-09-18 07:02:00 17 mins
I have plotted my data with this function:
plot(df$Start_Time,as.numeric(df$Duration), ylab = "Duration", xlab = "Date", ylim = c(0,450))
Since the data frame contains several tens of observations per day, I would like to draw a trend line in order to make it easier to read the data visually.
I tried this code:
fit <- glm(df$Start_Time~df$Duration)
co <- coef(fit)
abline(fit, col="red", lwd=2)
but I get this error:
Error in model.frame.default(formula = df$Start_Time ~ df$Duration, :
invalid type (list) for variable 'df$Start_Time'
I got the same error with this code:
abline(lm(df$Start_Time ~ df$Duration))
From reading the error messages, I suppose that those functions can't hande non-numeric values.
I tried this and got no error, but the line wasn't displayed on my graph:
fit <- glm(as.numeric(df$Start_Time)~df$Duration)
co <- coef(fit)
abline(fit, col="red", lwd=2)
What is the correct way of drawing trend lines / regression lines when one of the variables is in the datetime format?
NOTE: what follows is the result of str(df)
str(df)
'data.frame': 4121 obs. of 2 variables:
$ Start_Time: POSIXlt, format: "2014-09-18 10:01:00" "2014-09-18 08:01:00" "2014-09-18 08:01:00" "2014-09-18 08:01:00" ...
$ Duration :Class 'difftime' atomic [1:4121] 4 41 22 41 60 17 17 2 3 3 ... .. ..- attr(*, "units")= chr "mins"

Answer 1

Try the following code that reproduces data in the format you stated, then fits a linear model using lm() instead of glm() and plots the results, including a line of best fit.
set.seed(1)
times <- as.POSIXct("2014-09-18") + sort(runif(11, min=0, max=1000))
df <- data.frame(Start_time = times[-11])
df$Duration <- difftime(times[-11], times[-1])
model <- lm(Start_time ~ Duration, df)
plot(Start_time ~ Duration, df)
abline(model)
The structure of the data frame is the same as you report:
str(df)
'data.frame': 10 obs. of 2 variables:
$ Start_time: POSIXct, format: "2014-09-18 00:01:01" "2014-09-18 00:03:21" "2014-09-18 00:03:25" ...
$ Duration :Class 'difftime' atomic [1:10] -139.9 -4.29 -59.53 -106.62 -200.73 ...
.. ..- attr(*, "units")= chr "secs"