I'm not sure on the best way to filter out the extra line segment, it seems like a college level math problem, but the first thing that comes to mind is to just make a single line with multiple points. That way you just push the new waypoint onto the points for the line. Each waypath is just a single line with multiple points. This is just a simple example, but should show the idea. Like you, I can't get the mouse coordinates to line up. I'm totally confused on it and will ask on the maillist. It has something to do with groups and centering.

#!/usr/bin/perl
use warnings;
use strict;
use Gtk2 -init;
use Gnome2::Canvas;

my $window   = Gtk2::Window->new;
$window->signal_connect( destroy => sub { exit } );

my $scroller = Gtk2::ScrolledWindow->new;
my $canvas   = Gnome2::Canvas->new();
$scroller->add( $canvas );
$window->add( $scroller );
$window->set_default_size( 500, 500 );


# if scrollregion is smaller than default_size, a weird
# coordinate transform occurs
# $canvas->set_scroll_region( 0, 0, 400, 400 ); # bad
$canvas->set_scroll_region( 0, 0, 700, 700 ); # works good

$window->show_all;

my $root = $canvas->root;

# a stupid hack to try and compensate for bad scrollregion ;-(
#$root->move(-50,-50);  # dumb

my $text = Gnome2::Canvas::Item->new(
    $root, 'Gnome2::Canvas::Text',
   x          => 20,
   y          => 15,
   fill_color => 'black',
   font       => 'Sans 14',
   anchor     => 'GTK_ANCHOR_NW',
   text       => 'Click to add waypoint'
);

$canvas->signal_connect (event => \&event_handler);

my $points = [0,0,100,100];

my $line2= Gnome2::Canvas::Item->new ($root,
        'Gnome2::Canvas::Line',
         points => $points,
        fill_color => "red",
        width_units => 3.0,
        cap_style => 'projecting',
         join_style => 'miter',
        );

my $p = $line2->get('points');
print "@$p\n";
my ($x,$y) = @$p;

Gtk2->main;

##############################

sub event_handler{
     my ( $widget, $event ) = @_;
#     print $widget ,' ',$event->type,"\n";

    if ( $event->type eq "button-press" ) {
        print 'x->',$event->x,'  ','y->',$event->y,"\n";

        push @$points,$event->x , $event->y;

        $line2->set(points=>$points);

        my $p = $line2->get('points');
        print "@$p\n";
  
    }    
}

__END__
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