[R] centroid of diamond

[Rolf Turner]

Your question makes absolutely no sense at all.  See inline below.

On 14/03/14 08:03, al Vel wrote:
> Hello R users,
> I am trying to make a baricentric diagram like the ternary plot, but with 4
> edges. I want to know how to calculate the centroid of the diamond.

Which centroid?  A "diamond" (convex quadrilateral?) has 3 well defined 
centroids, in general all different.

> The 4 edges are A, B, C, D.

Are A, B, C, and D the *lengths* of the edges?  Or are they just labels 
for the edges?

> If value of A=B=C=D, then the point should be at the
> centre of the diamond. If A>B and B=C=D=0,

Since we are apparently talking about numerical values here it would 
seem that A, B, C, and D are the lengths of the sides.  How can you have 
a "diamond" (convex quadrilateral?) with 3 sides of length 0 and the 
other non-zero?

> Then the point should be at the corner of A.

What ***on earth*** does "the corner of A" mean?
>
> For diamond, how to convert the value of A,B,C,D into cartesian
> co-ordinates ?. if x1,x2,y1,y2 are A,B,C,D, then someone suggested:
>
>    new_point <- function(x1, x2, y1, y2, grad=1.73206){
>      b1 <- y1-(grad*x1)
>      b2 <- y2-(-grad*x2)
>      M <- matrix(c(grad, -grad, -1,-1), ncol=2)
>      intercepts <- as.matrix(c(b1,b2))
>      t_mat <- -solve(M) %*% intercepts
>      data.frame(x=t_mat[1,1], y=t_mat[2,1])
>    }
> But this is not working. Please do suggest some help.

Try using Google.  Wikipedia has a good article on quadrilaterals and 
outlines a procedure for finding the area centroid (I presume that's 
what you actually want) of a convex quadrilateral.

cheers,

Rolf Turner