Data Trending Expression

[Shock]

Hello,

If have a list of values being collected over time [value column]  [date column]

How can i trigger a warning to show Data is trending down over time

or, the current years values is less than last years value by 250

List of values  could could be 

350,1000,400   ( Warning)

or 350,1000,1500  ( no warning)

or 1200,900,500   ( Warning) etc etc    - this data trend is the ideal situation

Thank you for your help

[SkrOYC]

This is interesting and it's doable, can you share a real usecase?
Thinking out loud:

EOMONTH() - AppSheet Help
AVERAGE() - AppSheet Help
SELECT() - AppSheet Help

[Shock]

Yes . 
example : 

In 2019 a test was conducted on a water line to measure gallons per minute. The test result was 350 gpm. 

each year the test is repeated and a new gpm value collected . 
 
Overtime the values for most stay pretty consistent however, there are times a value may spike . These usually have an explanation like a new line was installed or perhaps the tester didn’t perform the test correctly. 
Other times the value may drop significantly or if analyzed , may show it has been dropping over time . This could also be a problem with the tester or it could indicate there is an issue with the line that needs looked into and maybe the line needs replaced . 

[Shock]

I found a way to do this. Not exactly what i wanted to do because i would rather avoid using virtual tables. 

Anyway ,  I set a virtual column [Analyze]to grabs the values in a list (all the test done for that ID)from another table.

I set another column to count how many values where in that [Analyze] column

After that i pulled the values from the different positions using index . Column for [Most recent Year], [Last Year] and [Two Years Ago]

I knew the count could vary so i set it the count to 3 to represent 3 years of data.

The final expression was successful in pulling data that has trending down over the last 3 years , or just 3 years worth of testing (most recent could be last year and worked backwards from there)

If(And([Flow tests] = 3,[Two years Ago] >[Last year],[Last year] > [Most Recent Year]), True, False)

That took forever