[igraph] [C] Drawing algorithms - coordinates question

[Vladimir Petrov]

Hello!

http://igraph.sourceforge.net/doc/html/ch14s01.html

"Layout generator functions (or at least most of them) try to place the
vertices  and  edges  of a graph on a 2D plane or in 3D space in a way
which visually pleases the human eye.

They  take  a graph object and a number of parameters as arguments and
return  an igraph_matrix_t, in which each row gives the coordinates of
a vertex."

Just want to precise:
1) In what units of measure result is represented? Pixels, inches or
some abstract units? For example, if i work with pixels, do i need just
set every parameter of the draw function (maxdelta, area, repulsrad
etc)? Is it enough or should i do some addition calculations?
2) What negatives values in the result matrix do mean? If there is
strong cartesian coordinates with 4 qusters, where the (0,0) point is?
Does "proot" vertex (one of the incoming parameters for the layout
funtion) have (0,0) coordinates?
3) How to calculate "area" parameter? In case of drawing on the
screens, is it right to just calculate the square multiplying height
pixels with weight pixels?
4) The most stupid question: after the drawing function complete, the
matrix x has a size of cols = a vertex number and rows = 2.
Does x[0,0] mean "x" coordinate and x[0,1] "y" coordinate for vertex 0?