[R] plotting every ith data point?

From: Jessi Brown <jessilbrown_at_gmail.com>
Date: Wed, 20 Feb 2008 18:57:31 -0500

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

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