DAX function to find MAXX of an expression to be used for x axis reference line

[antonyclootrack]

I have a dataset with columns brands, aspect, source, sentiment and sentence. I want to create a scatter plot with positivity on y axis and distinct count of sentences on the x axis. I am using DISTINCTCOUNT function on sentence column. I have also created a parameter containing columns brands, aspect and source. Used the paramter in value shelf, to determine the aggregation level for each point on the scatter plot. I need to create a reference line at 1/2 of the maximum distinct count of sentences for the paramter selection. How to create the function to be used in the constant x line function. 

[lbendlin]

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