DEVICE    |       READING       |       VALUE       |       TIMESTAMP
-----------------------------------------------------------------------------
 Thermometer |     temperature     |        20.0       |  2107.10.12 00:12:59
 Thermometer |     temperature     |        20.2       |  2107.10.12 00:24:12
                                  ...

SELECT
    filtered.hour as time,
    AVG((SELECT VALUE
        FROM history
        WHERE READING="temperature" AND DEVICE="Thermometer" AND TIMESTAMP <= filtered.hour
        ORDER BY TIMESTAMP DESC
        LIMIT 1
    )) as value
FROM (
    SELECT calculated.hour as hour FROM (
            SELECT DATE_ADD(DATE_SUB(DATE($__timeTo()), INTERVAL 10 YEAR), INTERVAL t4.i*10000 + t3.i*1000 + t2.i*100 + t1.i*10 + t0.i HOUR) as hour
            FROM (SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t0,
                     (SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t1,
                     (SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t2,
                     (SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t3,
                     (SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t4
    ) calculated
    WHERE calculated.hour >=  $__timeFrom() AND calculated.hour <= $__timeTo()
) filtered
GROUP BY DATE(filtered.hour)