SELECT group variation

Hello,
I start with influxdb and I need your expertise!
I save every 15 minutes in a series the evolution of a meter (gas)
I wish to make a request to retrieve the evolution of this meter hour by hour.
So I group the measurements by 1 hour and I apply the SPREAD function. But if the meter does not evolve for an entire hour, it is not enough:

Example:
I get this result

select SPREAD(value) from indexgaz where time > now() - 4h group by time(1h) fill(0)
name: indexgaz

time spread
1520020800000000000 0
1520024400000000000 0 –> desired value : 0.07
1520028000000000000 0
1520031600000000000 0
1520035200000000000 0

with these data :

select MIN(value) from indexgaz where time > now() - 4h group by time(1h) fill(0)
name: indexgaz
time min
1520020800000000000 12908.71
1520024400000000000 12908.78
1520028000000000000 12908.78
1520031600000000000 12908.78
1520035200000000000 0

SELECT MAX(value) from indexgaz where time > now() - 4h GROUP BY time(1h) fill(0)
name: indexgaz
time max
1520020800000000000 12908.71
1520024400000000000 12908.78
1520028000000000000 12908.78
1520031600000000000 12908.78
1520035200000000000 0

select COUNT(value) from indexgaz where time > now() - 4h group by time(1h) fill(0)
name: indexgaz
time count
1520020800000000000 4
1520024400000000000 4
1520028000000000000 4
1520031600000000000 4
1520035200000000000 0

Thank you for your advice

You need derivative function instead of spread. Spread shows the difference between max and min only inside each interval.
You may also take min or first value in each hour before the derivative.