Dynamic Column Label in Postgres 9.5

I am using postgres 9.5 and have built the following function:

CREATE or REPLACE FUNCTION func_getratio_laglag(_numeratorLAG text, _n1 int, _denominatorLAG text, _n2 int, _table text)
    RETURNS TABLE (date_t timestamp without time zone, customer_code text, index text, ratio real) AS
$BODY$
BEGIN
 RETURN QUERY EXECUTE 
        'SELECT 
    date_t,
    customer_code,
    index,
        (LAG('||quote_ident(_numeratorLAG)||',' || quote_literal(_n1)||') OVER W / LAG('||quote_ident(_denominatorLAG)||','|| quote_literal(_n2)||') OVER W) '
         || ' FROM ' || quote_ident(_table) 
         || ' WINDOW W AS (PARTITION BY customer_code ORDER BY date_t asc);';
END;
$BODY$ LANGUAGE plpgsql;

All the function does is allow me the ability to pick a 2 different columns from a specified table and calculate a ratio between them based on different lag windows. To execute the function above I use the following query:

Select * FROM func_getratio_laglag('order_first',1,'order_last',0,'customers_hist');

The output of this gives me a table with the column labels date_t, customer_code, index and ratio. I have really struggled on how to output ratio as a dynamic column label. That is, I would like to make it contingent on the input parameters e.g. if I ran the select query above then I would like the column labels date_t, customer_code, index and order_first_1_order_last_0.

I am stuck, any advice or hints?