MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

WITH heights AS (
    SELECT 1 AS id, 2 AS user_id, '2019-01-01 00:00:01' AS created_on, 180 AS height UNION ALL
    SELECT 2, 2, '2019-01-02 00:00:01', 181 UNION ALL
    SELECT 3, 3, '2019-01-03 00:00:01', 182 UNION ALL
    SELECT 4, 3, '2019-01-04 00:00:01', 183 UNION ALL
    SELECT 5, 2, '2019-01-07 00:00:01', 184
),
weights AS (
    SELECT 1 AS id, 2 AS user_id, '2019-01-01 00:00:01' AS created_on, 80 AS weight UNION ALL
    SELECT 2, 2, '2019-01-04 00:00:01', 81 UNION ALL
    SELECT 3, 3, '2019-01-05 00:00:01', 82 UNION ALL  
    SELECT 4, 3, '2019-01-06 00:00:01', 83 UNION ALL   
    SELECT 5, 2, '2019-01-07 00:00:01', 84
)

SELECT
    COALESCE(h.user_id, w.user_id) AS user_id,
    d.dt AS created_on,
    w.weight,
    h.height
FROM
(
    SELECT '2019-01-01 00:00:01' AS dt UNION ALL
    SELECT '2019-01-02 00:00:01' UNION ALL
    SELECT '2019-01-03 00:00:01' UNION ALL
    SELECT '2019-01-04 00:00:01' UNION ALL
    SELECT '2019-01-05 00:00:01' UNION ALL
    SELECT '2019-01-06 00:00:01' UNION ALL
    SELECT '2019-01-07 00:00:01'
) d
LEFT JOIN heights h
    ON d.dt = h.created_on AND h.user_id = 2
LEFT JOIN weights w
    ON d.dt = w.created_on AND w.user_id = 2
WHERE
    h.user_id IS NOT NULL OR w.user_id IS NOT NULL
ORDER BY
    d.dt;

user_id	created_on	weight	height
2	2019-01-01 00:00:01	80	180
2	2019-01-02 00:00:01	null	181
2	2019-01-04 00:00:01	81	null
2	2019-01-07 00:00:01	84	184