Constant y-axis value based on field value?

Hi MojoGene 
There is no option to work with your sample data file because all data is about the same point in time + the values are not equal to the values on the pictures....

in general, you can add constant lines in this way :
After creating a line graph :

and then format it.
Maximum line in the same way.

These lines are dynamic:

Pbix is attached.

More guides about working with constant lines :

https://www.easytweaks.com/add-horizontal-line-power-bi-chart-visual/

https://blogs.perficient.com/2023/05/03/analytics-pane-in-power-bi-part1/

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Hi MojoGene 
Now your sample has just one test ....if I understand correctly the table that you attached.

So I used again my file.

You can create customized constants. 
Ob the first step you need to create dimensions for date and lab tests with unique values.

These dimensions should be used as slicers and categories on the graph

Then you can create measures for constants :

sum_for_Max = if(SELECTEDVALUE('location_T'[Location],0)=0,"",

CALCULATE(max(space_missions[Price]),ALLSELECTED('calendar'[Date])))

 

sum_for_min = if(SELECTEDVALUE('location_T'[Location],0)=0,"",

CALCULATE(min(space_missions[Price]),ALLSELECTED('calendar'[Date])))

 

Result :

PBIX is attached

More information about dimension tables :

https://radacad.com/basics-of-modeling-in-power-bi-what-is-a-dimension-table-and-why-say-no-to-a-single-big-table

 

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Hi MojoGene 
You can work with every time granularity that you want, just create a time dimension table.
https://www.youtube.com/watch?v=-q7v56p192M&t=15s

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MojoGene 
You need to work with both date and time dimensions hierarchically:

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Hi MojoGene 
Glad to help.
According to the problems with the granularity, please post it as a new question, and attach a pbix with the model that you created with enough data (not one day, one lab, etc...) and expected results.