locate — mouse selection of a set of points
x=locate([n,flag])
locate is used to get the coordinates of one or more
points selected with the mouse in a graphics window. The coordinates
are given using the current graphics scale.
If n>0, n points are selected and their coordinates are returned in
the matrix x.
If n<=0, points are selected until the user clicks with the left button of the mouse which stands for stop. The last point (clicked with the left button) is not returned.
x=locate() is the same as x=locate(-1).
If flag=1 a cross is drawn at the points where the mouse is clicked.