Display total count in Power BI line chart

I think I got it figured out. My results won't match your data set because I had to recreate the sample data. It wouldn't let me copy and paste it from your doc because the size was too large. No worries though, I think I found a solution for you. First create a column as follows (you can add as many groups as you like, I only went to the last 24 months): 

6 Month Increments = 
VAR _DateDiff = 
    DATEDIFF(TODAY(),Sheet1[ModifiedDate],DAY)

RETURN
SWITCH(
    TRUE(),
    _DateDiff > (-180), "1. Last 6 Months",
    _DateDiff > (-360), "2. Last 12 Months",
    _DateDiff > (-540), "3. Last 18 Months",
    _DateDiff > (-720), "4. Last 24 Months",
    "5. Older"
)

Then create either use the quick measure creation tool I showed before, or just write one that mirrors the following: 

Count of CommunityID running total in 6 Month Increments = 
CALCULATE(
	COUNTA('Sheet1'[CommunityID]),
	FILTER(
		ALLSELECTED('Sheet1'[6 Month Increments]),
		ISONORAFTER('Sheet1'[6 Month Increments], MAX('Sheet1'[6 Month Increments]), DESC)
	)
)

This will give you the following result when you put them in your chart: 

Here is a PBIX with the solution in it as well in case it helps.