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the nodes from and to which the edge should be are specified. This
works well visually in the program code:

  $g->add_edge('London' => 'Paris');

Attributes such as 'label' can also be used. This specifies a label
for the edge.  The label can contain embedded newlines with '\n', as
well as '\c', '\l', '\r' for center, left, and right justified lines.

  $g->add_edge('London' => 'New York', label => 'Far');

Note that multiple attributes can be specified. Other attributes
include:

=over 4

=item minlen

sets an integer factor that applies to the edge length (ranks for
normal edges, or minimum node separation for flat edges)

=item weight

sets the integer cost of the edge. Values greater than 1 tend to
shorten the edge. Weight 0 flat edges are ignored for ordering
nodes

=item fontsize

sets the label type size in points

=item fontname

sets the label font family name

=item fontcolor

sets the label text colour

=item color

sets the line colour for the edge

A colour value may be "h,s,v" (hue, saturation, brightness) floating
point numbers between 0 and 1, or an X11 color name such as 'white',
'black', 'red', 'green', 'blue', 'yellow', 'magenta', 'cyan', or
'burlywood'

=item style

sets the style of the node. Can be one of: 'filled', 'solid',
'dashed', 'dotted', 'bold', 'invis'


=item dir

sets the arrow direction. Can be one of: 'forward', 'back', 'both',  'none'

=item tailclip, headclip

when set to false disables endpoint shape clipping

=item arrowhead, arrowtail

sets the type for the arrow head or tail. Can be one of: 'none',
'normal', 'inv', 'dot', 'odot', 'invdot', 'invodot.'

=item arrowsize

sets the arrow size: (norm_length=10,norm_width=5,
inv_length=6,inv_width=7,dot_radius=2)

=item headlabel, taillabel

sets the text for port labels. Note that labelfontcolor,
labelfontname, labelfontsize are also allowed

=item labeldistance, port_label_distance

sets the distance from the edge / port to the label. Also labelangle

=item decorate

if set, draws a line from the edge to the label

=item samehead, sametail

if set aim edges having the same value to the same port, using the
average landing point

=item constraint

if set to false causes an edge to be ignored for rank assignment

=back

Additionally, adding edges between ports of a node is done via the
'from_port' and 'to_port' parameters, which currently takes in the
offset of the port (ie 0, 1, 2...).

  $g->add_edge('London' => 'Paris', from_port => 0);

=cut

sub add_edge {
    my $self = shift;
    my $edge = shift;

    # Also cope with simple $from => $to
    if ( ref($edge) ne 'HASH' ) {
        my $from = $edge;
        my %edge = ( from => $from, to => shift, @_ );
        $edge = \%edge;
    }

    $self->add_edge_munge($edge) if $self->can('add_edge_munge');

    if ( not exists $edge->{from} or not exists $edge->{to} ) {
        carp("GraphViz add_edge: 'from' or 'to' parameter missing!");
        return;
    }