[R] plotting every ith data point?

[Jessi Brown]

Hello, fellow R enthusiasts.

Ok, I've been racking my brain about this small issue, and between
searching the help archives and reading through the plot-related
documentation, I can't figure out how to achieve my desired endpoint
without some ugly, brute force coding.

What I would like to do is make a plot in which only a subset of my
data are plotted, but in regular intervals, such as every 5th point
along the sequence. Is anyone aware of a built-in function in plot or
a related graphing family that can do this, or alternatively, a simple
way to extract the desired rows from my original dataframe? I want to
do this because I want to plot multiple series of points with their
confidence intervals (arrows), and even if I specify type="b," the
output ends up looking like just a series of crowded points.

For example, if you try making the plot below, you will see how
crowded two lines look without error bars:

> example.df<-data.frame(StartDate=(94:157), DSR1=seq(0.4, 0.8, length.out=64), DSR2=seq(0.3, 0.9, length.out=64))
> plot(example.df$StartDate, example.df$DSR1, type="b", ylim=c(0.3,0.9))
> points(example.df$StartDate, example.df$DSR2, type="b", pch=3)

Any ideas for an elegant solution to my dilemma?

Thanks in advance for any help.

cheers, Jessi Brown

Ph.D. student
Program in Ecology, Evolution, and Conservation Biology
University of Nevada, Reno