Trends using Counter as Log Type

[batistacaceres]

I have an object that increse its value each 15 min by 10: Example
Time    Value
00:00  -  0
00:15  -  10
00:30  - 20
00:45  - 30
01:00  - 40
00:15  - 50
00:30  - 60
00:45  - 70
02:00  - 80
.. and so on

Then, I have created a trend with that object, using the following parameters...
Log type: Counter
Agregate function: Last value
Trend resolution: 1 hour

I expect the Graph show me the diference between  the value in current hour and the last hour, for example
Time    Value              Trend Result that I expect
00:00  0                      -- >>  0
00:15  10         
...
01:00  40                    -- >> 40
00:15  50
...
02:00  80                    -- >> 40
00:15  50
.. and so on

but It always shows me cero values. Am I missunderstanding something??. What am I miss understanding about counter Log Type, or about the Agregate function???
What I want to do is, to graph (Trends) the diference in values between current hour and the last hour. Please could you help me??
I need to understand well How parameters "Log Type" and "Agregate function" work in the trend?? Is there a little documentation about it??

thank you very much
best regards