R help - Aug 2008 - Help with shading a polygon below a segment of a curve (normal distribution)

Dear R users,

I still feel new to R so please apologize if I am doing something stupid 
here. My use of the polygon() function produces a result that I cannot 
comprehend: In a plot, I would like to shade the area below a normal 
distribution. However, I do not want the entire area to be shaded, but 
just the area on the right side of a vertical line that I draw through 
the distribution (in order to illustrate the function of a t-test). Here 
is what I do:

scale <- 0.1
x <- seq(-4, 6, scale)
y <- dnorm(x)
plot(x, y, type = "l", main="t-Test, t = 2.2")

linepos <- 2.2
abline(v = linepos)

# I try to fill a polygon right of the vertical line:
# max(x) - linepos (in this case, 2.2) / scale (0.1)
# results in the last 38 elements of x and y.
# so I take the last 38 elements of x and y and try to
# draw a polygon underneath:

cutpoint <- (max(x) - linepos) / scale

xt <- x[(length(x)-cutpoint):length(x)]
yt <- y[(length(y)-cutpoint):length(y)]

# draw the polygon

polygon(xt, yt, density = 10 )

As you can see in the result, this is not what I want; some area above 
the line gets shaded, but not below. Can someone tell me what I am missing?

Thank you very much,
Bertolt

-- 
Bertolt Meyer
Oberassistent
Sozialpsychologie, Psychologisches Institut der Universit?t Z?rich
Binzm?hlestr. 14, Box 15
CH-8050 Z?rich

bmeyer at sozpsy.uzh.ch
tel:   +41446357282
fax:   +41446357279
mob:   +41788966111

Bertolt Meyer wrote:
> Dear R users,
>
> I still feel new to R so please apologize if I am doing something
> stupid here. My use of the polygon() function produces a result that I
> cannot comprehend: In a plot, I would like to shade the area below a
> normal distribution. However, I do not want the entire area to be
> shaded, but just the area on the right side of a vertical line that I
> draw through the distribution (in order to illustrate the function of
> a t-test). Here is what I do:
>
> scale <- 0.1
> x <- seq(-4, 6, scale)
> y <- dnorm(x)
> plot(x, y, type = "l", main="t-Test, t = 2.2")
>
> linepos <- 2.2
> abline(v = linepos)
>
> # I try to fill a polygon right of the vertical line:
> # max(x) - linepos (in this case, 2.2) / scale (0.1)
> # results in the last 38 elements of x and y.
> # so I take the last 38 elements of x and y and try to
> # draw a polygon underneath:
>
> cutpoint <- (max(x) - linepos) / scale
>
> xt <- x[(length(x)-cutpoint):length(x)]
> yt <- y[(length(y)-cutpoint):length(y)]
>
> # draw the polygon
>
> polygon(xt, yt, density = 10 )
>
> As you can see in the result, this is not what I want; some area above
> the line gets shaded, but not below. Can someone tell me what I am
> missing?
>
> Thank you very much,
> Bertolt
>
Your polygon contains no points on the x axis (yt==0).

Try adding this:

n <- length(xt)
xt <- c(xt[1], xt, xt[n])
yt <- c(0,yt,0)
polygon(xt, yt, col="red" )




-- 
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907

library(HH)
normal.and.t.dist()

There are many related examples in
example(normal.and.t.dist)

Bertolt,

The points you send to polygon() do not fully enclose the area you desire.  Try
adding one more point as such

xt <- c(x[(length(x)-cutpoint):length(x)],linepos)
yt <- c(y[(length(y)-cutpoint):length(y)],0)
polygon(xt, yt, density = 10 )


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Bertolt Meyer
Sent: Thursday, August 28, 2008 9:03 AM
To: r-help at r-project.org
Subject: [R] Help with shading a polygon below a segment of a curve
(normaldistribution)

Dear R users,

I still feel new to R so please apologize if I am doing something stupid 
here. My use of the polygon() function produces a result that I cannot 
comprehend: In a plot, I would like to shade the area below a normal 
distribution. However, I do not want the entire area to be shaded, but 
just the area on the right side of a vertical line that I draw through 
the distribution (in order to illustrate the function of a t-test). Here 
is what I do:

scale <- 0.1
x <- seq(-4, 6, scale)
y <- dnorm(x)
plot(x, y, type = "l", main="t-Test, t = 2.2")

linepos <- 2.2
abline(v = linepos)

# I try to fill a polygon right of the vertical line:
# max(x) - linepos (in this case, 2.2) / scale (0.1)
# results in the last 38 elements of x and y.
# so I take the last 38 elements of x and y and try to
# draw a polygon underneath:

cutpoint <- (max(x) - linepos) / scale

xt <- x[(length(x)-cutpoint):length(x)]
yt <- y[(length(y)-cutpoint):length(y)]

# draw the polygon

polygon(xt, yt, density = 10 )

As you can see in the result, this is not what I want; some area above 
the line gets shaded, but not below. Can someone tell me what I am missing?

Thank you very much,
Bertolt

-- 
Bertolt Meyer
Oberassistent
Sozialpsychologie, Psychologisches Institut der Universit?t Z?rich
Binzm?hlestr. 14, Box 15
CH-8050 Z?rich

bmeyer at sozpsy.uzh.ch
tel:   +41446357282
fax:   +41446357279
mob:   +41788966111

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

The power.examp function in the TeachingDemos package (among others) may already
do what you want.  Even if it does not, it does shade the area under a curve,
you can look at the source for the function as an example of where to start.

Hope this helps,

--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
(801) 408-8111


> -----Original Message-----
> From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org] On Behalf Of Bertolt Meyer
> Sent: Thursday, August 28, 2008 8:03 AM
> To: r-help at r-project.org
> Subject: [R] Help with shading a polygon below a segment of a
> curve (normal distribution)
>
> Dear R users,
>
> I still feel new to R so please apologize if I am doing
> something stupid here. My use of the polygon() function
> produces a result that I cannot
> comprehend: In a plot, I would like to shade the area below a
> normal distribution. However, I do not want the entire area
> to be shaded, but just the area on the right side of a
> vertical line that I draw through the distribution (in order
> to illustrate the function of a t-test). Here is what I do:
>
> scale <- 0.1
> x <- seq(-4, 6, scale)
> y <- dnorm(x)
> plot(x, y, type = "l", main="t-Test, t = 2.2")
>
> linepos <- 2.2
> abline(v = linepos)
>
> # I try to fill a polygon right of the vertical line:
> # max(x) - linepos (in this case, 2.2) / scale (0.1) #
> results in the last 38 elements of x and y.
> # so I take the last 38 elements of x and y and try to # draw
> a polygon underneath:
>
> cutpoint <- (max(x) - linepos) / scale
>
> xt <- x[(length(x)-cutpoint):length(x)]
> yt <- y[(length(y)-cutpoint):length(y)]
>
> # draw the polygon
>
> polygon(xt, yt, density = 10 )
>
> As you can see in the result, this is not what I want; some
> area above the line gets shaded, but not below. Can someone
> tell me what I am missing?
>
> Thank you very much,
> Bertolt
>
> --
> Bertolt Meyer
> Oberassistent
> Sozialpsychologie, Psychologisches Institut der Universit?t
> Z?rich Binzm?hlestr. 14, Box 15 CH-8050 Z?rich
>
> bmeyer at sozpsy.uzh.ch
> tel:   +41446357282
> fax:   +41446357279
> mob:   +41788966111
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>