[R] Help with shading a polygon below a segment of a curve (normal distribution)

[Peter Dalgaard]

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" )




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