Calculate difference of values from the same metric

Grafana Dashboards
1
  • What Grafana version and what operating system are you using?

Grafana v9.5.21 (2dd6ebef03fbe42927769bf00bcf0bc4e621bb9c)
Ubuntu 18
DataSource - VictoriaMetrics v1.84.0

  • What are you trying to achieve?

calculate difference between last value and last but one value of metric and show the result on dashboard.

for example I have such data:

02-01-25_08-01-26: 0.21: 
01-01-25_16-00-01: 0.18: 
01-01-25_08-03-13: 0.19:

I want to calculate 0.21-0.18 == 0.03 and have this value 0.03 in the new field so I can show this in the Stat panel for example.

  • How are you trying to achieve it?

the last value 0.21 I can get with this query:

last_over_time(snow_depth{srv_ID="4"}[1d])*100

so this is my first question:
how to get value 0.18?

lets assume that data comes to datasource 2 times per day - around 8AM and 4PM.
(but for other metric which I also want to do same calculation - pattern would be - every half of hour)

Also there is second part of my task - calculating the difference of values.
I checked Transform “Add field from calculation” but seems there is some issue with creating new field from binary operation on two different fields - it will create new field with result only if the source fields - are the same!
and it does not create new field with result of calculation if the source fields different!
it is really strange.
Please take a look on the first picture - fields in calculation the same - SRV4_snow_depth and it have created test_field123 - so I can use it to show on the panel:

image
image1897Ă—531 63.5 KB

but when I use different fields - it shows that there no such field test_field123

image
image1897Ă—535 64.8 KB

And here my second question - where Im wrong to use this Transform? Should it be done in some different way?

2

Hi,
I’m not sure but maybe idelta function would help? (I’m not sure if you have high enough version of VM but since it’s supported by prometheus, I think you should be fine).

1 Like
3

“idelta” seems works but not sure for its “lookbehind window d”
in one my case, when data changes 2 times per day - 8 AM and 4PM, I have use this query:

idelta(snow_depth{srv_ID="4"}[1d])*100 to receive correct difference between last and last but one values

despite the fact that between two metric values it is only 8h I need to use 1d to receive correct result. I dont know how it works and when it possible to get the wrong result.

So I still would like to know - how to get value 0.18 ? how to get the “last but one” value?