[R] matrix and error in points, plot ?

[Uwe Ligges]

Patrick Giraudoux H wrote:

> Hi,
> 
> Would a specialist of the "point" or "plot" functions try the following:
> 
> mat<-matrix(c(1.836767,4.025989,0.6396777,0.3764444),ncol=2)
> plot(mat)
> points(mat[1,],col="red")
> 
> ..A lag appears on x for mat [1,1] between the two displays.
> 
> I wonder if this example may be due to a bug or to the mis-use of a 
> matrix in the plot() points()  functions. In case of mise-use which kind 
> can it be?


It's misuse:

For a matrix, the first column is interpreted as the x-coordinates, the 
second one as y-coordinates.

For a vector (mat[1,] is a (1D) vector!) plot/points plots the values 
(as y-coordinates) against their index (1:2 in this case, where 1 is not 
visible in the plot).

You can use
   points(mat[1,,drop=FALSE], col="red")
in order not to drop dimensions, so mat[1,,drop=FASLE] is still a 
2-column matrix.

Uwe Ligges



> I am working with R 2.0.1 and Windows XP.
> 
> Cheers,
> 
> Patrick Giraudoux
> 
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