Incrementally add seconds of a timestamp column grouped by ID in R - r

Question

I have a dataframe that is essentially a time series data.
Timestamp <- c("1/27/2015 18:28:16","1/27/2015 18:28:17","1/27/2015 18:28:19","1/27/2015 18:28:20","1/27/2015 18:28:23","1/28/2015 22:43:08","1/28/2015 22:43:09","1/28/2015 22:43:13","1/28/2015 22:43:15","1/28/2015 22:43:16"
)
ID <- c("A","A","A","A","A","B","B","B","B","B")
v1<- c(1.70,1.71,1.77,1.79,1.63,7.20,7.26,7.16,7.18,7.18)
df <- data.frame(Timestamp ,ID,v1)
Timestamp ID v1
1/27/2015 18:28:16 A 1.70
1/27/2015 18:28:17 A 1.71
1/27/2015 18:28:19 A 1.77
1/27/2015 18:28:20 A 1.79
1/27/2015 18:28:23 A 1.63
1/28/2015 22:43:08 B 7.20
1/28/2015 22:43:09 B 7.26
1/28/2015 22:43:13 B 7.16
1/28/2015 22:43:15 B 7.18
1/28/2015 22:43:16 B 7.18
Since I dont really care about the timestamp, I was thinking of creating a column called interval to plot this data in one plot.
I am wrongly creating the interval column by doing this
df$interval <- cut(df$Timestamp, breaks="sec")
I want to incrementally add the "secs" of the timestamp and put it in the interval column and this should by grouped by ID. By this I mean, Everytime it has a new ID, the interval column resets to 1 and then incrementally adds the timestamp (secs).
My desired output
Timestamp ID v1 Interval
1/27/2015 18:28:16 A 1.70 1
1/27/2015 18:28:17 A 1.71 2
1/27/2015 18:28:19 A 1.77 4
1/27/2015 18:28:20 A 1.79 5
1/27/2015 18:28:23 A 1.63 8
1/28/2015 22:43:08 B 7.20 1
1/28/2015 22:43:09 B 7.26 2
1/28/2015 22:43:13 B 7.16 6
1/28/2015 22:43:15 B 7.18 8
1/28/2015 22:43:16 B 7.18 9
I also would like to plot this using ggplot with interval vs v1 by ID and so we get 2 time series in the same plot. I will then extract features from it.
Please help me how to work around this problem so that I can apply it to a larger dataset.

Answer 1

One solution with data.table:
For the data:
library(data.table)
df <- as.data.table(df)
df$Timestamp <- as.POSIXct(df$Timestamp, format='%m/%d/%Y %H:%M:%S')
df[, Interval := as.numeric(difftime(Timestamp, .SD[1, Timestamp], units='secs') + 1) , by=ID]
which outputs:
> df
Timestamp ID v1 Interval
1: 2015-01-27 18:28:16 A 1.70 1
2: 2015-01-27 18:28:17 A 1.71 2
3: 2015-01-27 18:28:19 A 1.77 4
4: 2015-01-27 18:28:20 A 1.79 5
5: 2015-01-27 18:28:23 A 1.63 8
6: 2015-01-28 22:43:08 B 7.20 1
7: 2015-01-28 22:43:09 B 7.26 2
8: 2015-01-28 22:43:13 B 7.16 6
9: 2015-01-28 22:43:15 B 7.18 8
10: 2015-01-28 22:43:16 B 7.18 9
Then for ggplot:
library(ggplot2)
ggplot(df, aes(x=Interval, y=v1, color=ID)) + geom_line()
and the graph: