[R] Plotting a quadratic line on top of an xy scatterplot

From: Josh B <joshb41_at_yahoo.com>
Date: Mon, 11 Apr 2011 12:29:09 -0700 (PDT)


Dear Listserv,

Here is my latest in a series of simple-seeming questions that dog me.

Consider the following data:

x <- read.table(textConnection("temperature probability 0.11 9.4
0 2.3

0.38 8.7
0.43 9.2
0.6 15.6
0.47 8.7
0.09 12.8
0.11 9.4
0.01 7.7
0.83 8
0.65 9.3
0.05 7.4
0.34 10.1
0.02 4.8
0.07 9.1
0.6 15.6
0.01 8.4
0.9 9.6
0.83 8
0.12 8.4
0.01 8

0 5
0.11 9.7
0.41 7.4
0.05 9.4
0.09 8.3

0 6.1
0.12 8.4
0.73 7.8
0 4.2"), header = TRUE, as.is = TRUE)
closeAllConnections()

I modeled the relationship: Probability = f(Temperature), i.e., probability as a function of temperature.

I found that there is a significant quadratic term in the model:

summary(lm(x[,2] ~ x[,1] + I(x[,1]^2)))

Now the question is: how do I plot it?

I can do this:
plot(x[,2] ~ x[,1])

...but I would also like to add a line corresponding to the quadratic function. In other words, I want to visually show the relationship among the variables that is being modeled. How do I do it? I think the curve() command will be used, but I don't know how to employ it.

Thanks very much in advance.

Sincerely,



Josh Banta, Ph.D
Center for Genomics and Systems Biology
New York University
100 Washington Square East
New York, NY 10003
Tel: (212) 998-8465
http://plantevolutionaryecology.org

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