Need to subtract two matrix values to populate a third visual in Power BI Desktop

[CMccown]

I am new to Power BI desktop and DAX so I need a bit of help with a problem.

I have two matrix visuals in a monthly page that reflect average monthly temperatures.

The underlying data that populates the Actual Monthly Temperatures matrix is stored in a table containing daily temperatures (raw data). The data in the Normal Temperatures matrix is stored in a separate table and is already aggregated (these are averaged values). 

I need to calculate the monthly value for above or below normal for each column shown in each matrix for a third visual depicting temperatures above or below normal. In other words for January: 55.9 - 58.3 = 2.7 (for HiTemp) , 39.0-39.2 = -.2 (for LoTemp) etc.

I understand that I can aggregate the daily data in a measure, but I am not sure how to subtract a regular column value from a separate table (Normal Temperatures - values already aggregated) from a measure.

Any help would be greatly appreciated!

****UPDATE****

This issue was resolved by updating the underlying model to create a date table and a dimension table for the month value as this was the only column that the aggregated and unaggregated data had in common. After creating proper relationships, measures calculating totals were created. 

@TheoC  Thank you for your time!

[CMccown]

Apologies Theo, yes January would be 56.9-59.3= -2.4 Hi Temp, 41.5-41.5=0 for Lo temp etc. I think when I listed the values I had sliced the data to a separate weather station and those were the values I had in the text.

[TheoC]

Hi @CMccown 

Thanks for letting me know. 

Can you provide answers to the other questions as well please?  

Thanks,
Theo

[TheoC]

Hi @CMccown 

I'm trying to better understand your data.  You've listed January as 55.9 - 58.3 = 2.7 (for HiTemp) however January has 59.3 and 56.9 per the diagrams.  Or is it that I have completely misunderstood where the figures have come from?  If I haven't, does that mean that the 2.7 should be 2.4?  Similarly, Lo Temp for January in the diagrams is 41.5 and 41.5 - but again, I may have misunderstood where these numbers are coming from.  

A few other questions before I can provide a solution:

• Do you have a Date table established in your data model?
• If there is no Date table, is there a relationship between the two sets of data? If so, what is the relationship?
• Can you provide your data sets or some dummy data?

Many thanks and look forward to hearing from you soon!

Theo 🙂

[CMccown]

Hi Theo,

Below  are two screen shots, one of the model and one of the data for the Normal table, just to show how the dates for Normal are presented, they are cumulative for the time period. I do not have a date table and the relationship I have is by month. 

Thank you again for your time.

Candace