Plotting accumulated data

Hello all

I have the data frame with this:

            ID              DATETIME     TRN      TRN_S
1 1192756 2010-06-23 15:39:07 13.420 0.2236667
2 1192757 2010-06-23 15:40:07 13.805 0.2300833
3 1192758 2010-06-23 15:41:07 13.860 0.2310000
4 1192759 2010-06-23 15:42:07 13.750 0.2291667
5 1192760 2010-06-23 15:43:07 13.530 0.2255000
6 1192761 2010-06-23 15:44:07 13.805 0.2300833

Now, I would like to plot a graph with DATETIME and the accumulated
TRN_S.

I can run a while loop like this:

accumulated_trns <- 0
last_value <-length(data$TRN_S)
while (i < last_value) {
acc_trns <- acc_trns + data$TRN_S[i]
data$accumulated_trns[i] <- acc_trns
i <- i + 1
}
data$accumulated_trns[i] <- acc_trns
plot(data$DATETIME, data$accumulated_trns)

This is slow and demanding on the computer.

There must be a better way to accumulate values! Any suggestion  
greatly
appreciated.

	[[alternative HTML version deleted]]