[R] Using nrow with summaryBy

Hello Everyone- I'm calculating summary statistics on a dataset (~4000 records, observations are not uniformly distributed) using summaryBy and trying to add a column with the number of observations to the output as well. What occurs to me is to use nrow(), but this doesn't appear to be working

[R] Plotting extra lines on scatterplot

Hello Everyone- I'm in the process of slowly learning R and am having a little bit of trouble plotting an extra line onto a scatterplot. I'm sure the answer is quite simple but I am stumped. The code I am using is: headways - read.table(headways.csv, header=TRUE, sep=,, na.strings=, dec=.,