Mysql count current row count group by date

It is not as easy as just use a group by, but as you thought group by is important to do this. I guess there will be no simple 5 line query to get your expected result.
I would suggest to do multiple queries and some backend code instead of a single query. It is hard to read, understand and maintain.

But it is possible.

A GROUP BY DATE_FORMAT(Created,'%Y-%m-%d') in a simple select would already give you the count per day.

The next problem to solve is, that you want the days_back instead of the date itself. Thats also quite simple with DATEDIFF(DATE_FORMAT(NOW(),'%Y-%m-%d'), DATE_FORMAT(Createdate,'%Y-%m-%d')).

But the third requirement is the difficult. You don’t want the amount per day but the sum up to that day, for a variable amount of days back. Unfortunatly, mysql does not offer a Sequence (like SELECT numbers from 1 to X) to join with the other parts of the query. If you can live with a restriction to somewhat about 300 years, https://stackoverflow.com/a/9296238/4675841 will help there.
To get article counts and user counts together, you coult either use union (all) or a join, I used the join and came up with this query.

SELECT user_query.diff as days_back, article_count as count, user_count as users
FROM (
    SELECT DATEDIFF(DATE_FORMAT(NOW(),'%Y-%m-%d'), gen_date) as diff, COUNT(create_date) as user_count
    FROM (
        SELECT adddate('1970-01-01',t4*10000 + t3*1000 + t2*100 + t1*10 + t0) gen_date
        FROM
            (SELECT 0 t0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t0,
            (SELECT 0 t1 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t1,
            (SELECT 0 t2 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t2,
            (SELECT 0 t3 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t3,
            (SELECT 0 t4 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t4) possible_dates
        LEFT JOIN (SELECT DATE_FORMAT(Createdate,'%Y-%m-%d') as create_date FROM tbl_users) counts
            ON create_date <= gen_date
        WHERE gen_date BETWEEN DATE_SUB(NOW(), INTERVAL 20 DAY) AND NOW()
        GROUP BY gen_date) user_query
INNER JOIN (
    SELECT DATEDIFF(DATE_FORMAT(NOW(),'%Y-%m-%d'), gen_date) as diff, COUNT(create_date) as article_count
    FROM (
        SELECT adddate('1970-01-01',t4*10000 + t3*1000 + t2*100 + t1*10 + t0) gen_date
        FROM
            (SELECT 0 t0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t0,
            (SELECT 0 t1 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t1,
            (SELECT 0 t2 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t2,
            (SELECT 0 t3 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t3,
            (SELECT 0 t4 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t4) v
        LEFT JOIN (SELECT DATE_FORMAT(Createdate,'%Y-%m-%d') as create_date FROM tbl_articles) counts
            ON create_date <= gen_date
        WHERE gen_date BETWEEN DATE_SUB(NOW(), INTERVAL 20 DAY) AND NOW()
    GROUP BY gen_date) article_query
    ON user_query.diff = article_query.diff
ORDER BY days_back

Try it out.