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I would create a DAX table with the following script and sort the label column by the order. Then you'll need to create a relationship to the source with the values Low, Medium, High. After that, change the column to your matrix to the "Label" from "Chart Label List". The other way is adding spaces to the labels e.g. "  Low", " Medium", "High". I wouldn't recommend this.

Chart Label List = 
VAR _result = 
	DATATABLE (
      "Label"	, STRING
    , "Order"	, INTEGER
	,
    {
          {"Low", 1}
		, {"Medium", 2}
		, {"High", 3}
    }
)

RETURN
_result