Viola a restrição de chave estrangeira

Estou tentando implementar a UML abaixo e não consigo descobrir onde está meu erro, pois já coloquei um valor na tabela.

-- Schema: public

DROP SCHEMA IF EXISTS public CASCADE;

CREATE SCHEMA public
  AUTHORIZATION postgres;
GRANT ALL ON SCHEMA public TO postgres;
GRANT ALL ON SCHEMA public TO public;
COMMENT ON SCHEMA public IS 'standard public schema';

CREATE TYPE color AS ENUM ('green','black','yellow','grey','blue');
CREATE TYPE arrowtype AS ENUM ('both_arrows','begin_arrow','end_arrow'); 
CREATE TYPE treetype AS ENUM ('orange_tree','apple_tree');
CREATE TYPE linetype AS ENUM ('hairline','solid','dotted','dashed');

CREATE TYPE address AS (
street_name TEXT,
street_no TEXT
);

CREATE TYPE square AS (
row VARCHAR(10),
colmn VARCHAR(10)
);

CREATE TABLE user_table (
username VARCHAR(30) NOT NULL PRIMARY KEY,
password VARCHAR(16),
name     VARCHAR(100),
lastname VARCHAR(100),
telephone VARCHAR(20)[],
address address,
birth_year INTEGER
);

CREATE TABLE GraphPanel (
id SERIAL NOT NULL PRIMARY KEY,
title VARCHAR(255),
creation_date TIMESTAMP,
comments TEXT
);

CREATE TABLE GraphObject (
id SERIAL NOT NULL PRIMARY KEY,
color color,
title VARCHAR(255)
);

CREATE TABLE House (
roof_color color,
roof_height INTEGER,
position square
)INHERITS(GraphObject);

CREATE TABLE Arrow (
arrow_line linetype,
arrow_type arrowtype
)INHERITS(GraphObject);

CREATE TABLE Tree (
position square,
tree_type treetype
)INHERITS(GraphObject);

CREATE TABLE creator (
username VARCHAR(30),
panel INTEGER,
category VARCHAR(50),
FOREIGN KEY (username) REFERENCES user_table(username),
FOREIGN KEY (panel) REFERENCES GraphPanel(id)
);

CREATE TABLE container (
panel  INTEGER,
gobject INTEGER,
FOREIGN KEY (panel) REFERENCES GraphPanel(id),
FOREIGN KEY (gobject) REFERENCES GraphObject(id)
);

-- INSERTING VALUES 

-- Users

INSERT INTO user_table (
    username,
    password,name,
    lastname,
    telephone,
    address,
    birth_year
) VALUES (
    'giannis',
    'thepassword',
    'Giannis',
    'Christofakis',
    '{"6944789333","2831051300"}',
    ROW('Evridikis','7'),
    1985
);

INSERT INTO user_table (
    username,
    password,
    name,
    lastname,
    telephone,
    address,
    birth_year
) VALUES (
    'antonis',
    'psw',
    'Antonis',
    'Papadakis',
    '{"6974246700","2831051312"}',
    ROW('Doukos Mpofor','12'),
    1993
);

INSERT INTO user_table (
    username,
    password,
    name,
    lastname,
    telephone,
    address,
    birth_year
) VALUES (
    'mpampis',
    'thepsw',
    'Mpampis',
    'Theodorou',
    '{"6984652333","284131100"}',
    ROW('Markoy Portaliou','8'),
    1981
);

-- SELECT * FROM user_table;

-- GraphPanels

INSERT INTO GraphPanel (
    title,
    creation_date,
    comments
) VALUES (
    'Platia Sinani',
    '2004-10-19 10:23:54',
    'Diamorfosi ths platias Sinani'
);

INSERT INTO GraphPanel (
    title,
    creation_date,
    comments
) VALUES (
    'Parking TEI',
    '2008-11-04 22:20:01',
    'Apeikonisi parking tou TEI'
);

INSERT INTO GraphPanel (
    title,
    creation_date,
    comments
) VALUES (
    'Perioxh Stavormenou',
    '2011-05-17 18:23:33',
    'Dimhourgeia parkou sth perioxh tou Estavromenou'
);

INSERT INTO GraphPanel (
    title,
    creation_date,
    comments
) VALUES (
    'Odos Panagiotaki',
    '2000-09-22 11:00:33',
    'Kataskeuh gefuras'
);

-- SELECT * FROM GraphPanel;

-- creator

INSERT INTO creator (
    username,
    panel,
    category
) VALUES (
    'giannis',
     1,
    'Diamorfosh'
);

INSERT INTO creator (
    username,
    panel,
    category
) VALUES (
    'mpampis',
     2,
    'Apeikonish'
);

INSERT INTO creator (
    username,
    panel,
    category
) VALUES (
    'antonis',
     3,
    'Kataskeuh'
);

INSERT INTO creator (
    username,
    panel,
    category
) VALUES (
    'giannis',
     4,
    'Kataskeuh'
);

-- SELECT * FROM creator;

-- GraphObjects

-- House

INSERT INTO HOUSE (
    color,
    title,
    roof_color,
    roof_height,
    position
) VALUES (
    'grey',
    'Ate Bank',
    'green',
     6,
     ROW('12','A3')
);

INSERT INTO HOUSE (
    color,
    title,
    roof_color,
    roof_height,
    position
) VALUES (
    'grey',
    'residents house',
    'blue',
     4,
     ROW('23','Y3')
);

INSERT INTO HOUSE (
    color,
    title,
    roof_color,
    roof_height,
    position
) VALUES (
    'blue',
    'Super Market',
    'yellow',
     7,
     ROW('08','L2')
);

-- SELECT * FROM House;

-- Trees

INSERT INTO Tree (
    color,
    title,
    position,
    tree_type
) VALUES (
    'green',
    'valencia',
     ROW('07','L2'),
     'orange_tree'
);

INSERT INTO Tree (
    color,
    title,
    position,
    tree_type
) VALUES (
    'green',
    'mantarinia',
     ROW('18','J2'),
     'orange_tree'
);

INSERT INTO Tree (
    color,
    title,
    position,
    tree_type
) VALUES (
    'green',
    'firikia',
     ROW('03','E1'),
     'apple_tree'
);

-- SELECT * FROM Tree;

INSERT INTO Arrow (
    color,
    title,
    arrow_line,
    arrow_type
) VALUES (
    'black',
    'starting point',
     'solid',
     'begin_arrow'
);

INSERT INTO Arrow (
    color,
    title,
    arrow_line,
    arrow_type
) VALUES (
    'grey',
    'ending way',
     'dashed',
     'end_arrow'
);

INSERT INTO Arrow (
    color,
    title,
    arrow_line,
    arrow_type
) VALUES (
    'black',
    'both ways',
     'dotted',
     'both_arrows'
);

 SELECT * FROM GraphObject order by id;

-- container



INSERT INTO container (
    panel,
    gobject
) VALUES (
     1,
     6
);


SELECT * FROM container;

-- SELECT table_name FROM information_schema.tables WHERE table_schema = 'public';

O erro está relacionado à última parte do código.

ERRO: inserção ou atualização na tabela "container" viola a restrição de chave estrangeira "container_gobject_fkey" DETALHE: Chave (gobject)=(6) não está presente na tabela "graphobject".

********** Erro **********

ERRO: inserção ou atualização na tabela "container" viola a restrição de chave estrangeira "container_gobject_fkey" Estado SQL: 23503 Detalhe: Chave (gobject)=(6) não está presente na tabela "graphobject".

ATUALIZAR

OK, eu não entendi herança. Alguém pode me aconselhar como implementar a containsrelação entre GraphPanele GraphObject, Tree, Arrow, House.