xchange transform real to pixel coordinates

## Syntax

    [x1, y1, rect] = xchange(x, y, dir)

## Arguments

x, y

  two matrices of size `(n1, n2)` (coordinates of a set of points).

dir

  a string, parameter used to specify the conversion type (See "Description" for details).

x1, y1

  two matrices of size `(n1, n2)` (coordinates of the set of points).

rect

  a vector of size 4.

## Description

After having used a graphics function, `xchange` computes pixel coordinates from real coordinates and conversely, according to the value of the parameter `dir`: `"f2i"` (float to int) means real to pixel and `"i2f"` (int to float) means pixel to real.

`x1` and `y1` are the new coordinates of the set of points defined by the old coordinates `x` and `y`.

`rect` is the coordinates in pixel of the rectangle in which the plot was done: `[upper-left point, width, height]`.

## Examples

    t=[0:0.1:2*%pi]';
    plot2d(t,sin(t))
    [x,y,rect]=xchange(1,1,"f2i")
    [x,y,rect]=xchange(0,0,"i2f")