[R] linear trend line and a quadratic trend line.

[ONKELINX, Thierry]

Here is a solution using ggplot2

n <- 50
Duncan <- data.frame(income = runif(n, 0, 50000))
Duncan$prestige <- with(Duncan, 0.01 * income - .0000001 * income ^2 +
rnorm(n, sd = 10))

library(ggplot2)
ggplot(Duncan, aes(y = prestige, x = income)) +
#define the data
		geom_point() +
#add the data points
		geom_smooth(method = "lm", colour = "red") +
#add a linear trend
		geom_smooth(method = "lm", formula = y ~ poly(x, 2),
color = "green") #add the quadratic trend

HTH,

Thierry


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Eric Fail
Verzonden: zondag 8 november 2009 18:21
Aan: r-help at r-project.org
Onderwerp: [R] linear trend line and a quadratic trend line.




Dear list users



How is it possible to visualise both a linear trend line and a quadratic
trend line on a plot of two variables?



Here my almost working exsample.



data(Duncan)

attach(Duncan)

plot(prestige ~ income)

abline(lm(prestige ~ income), col=2, lwd=2)



Now I would like to add yet another trend line, but this time a
quadratic one. So I have two trend lines. One linear trend line and a
quadratic trend line. A trend line as if I had taken
I(income^2) into the equation.



I know I can make two models and compare them using anova, but for
pedagogical resons I wold like to visualise it. I know the trick from my
past as an SPSS user, but I would like to do this in R as well. Se it in
SPSS
http://www.childrens-mercy.org/stats/weblog2006/QuadraticRegression.asp



Thanks in adcance



Eric

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