Recursive CTE hangs - Mandelbrot set calculation

WITH RECURSIVE

  xaxis(x) AS (VALUES(-2.0) UNION ALL SELECT x+0.05 FROM xaxis WHERE x<1.2),

  yaxis(y) AS (VALUES(-1.0) UNION ALL SELECT y+0.1 FROM yaxis WHERE y<1.0),

  m(iter, cx, cy, x, y) AS (

    SELECT 0, x, y, 0.0, 0.0 FROM xaxis, yaxis

    UNION ALL

    SELECT iter+1, cx, cy, x*x-y*y + cx, 2.0*x*y + cy FROM m 

     WHERE (x*x + y*y) < 4.0 AND iter<28

  ),

  m2(iter, cx, cy) AS (

    SELECT max(iter), cx, cy FROM m GROUP BY cx, cy

  ),

  a(t) AS (

    SELECT group_concat( substr(' .+*#', 1+least(iter/7,4), 1), '') 

    FROM m2 GROUP BY cy

  )

SELECT group_concat(rtrim(t),x'0a') FROM a;