-- hours 0 - 23
create temporary table hours_t (h int, today int, yesterday int);
insert into hours_t(h)  values
  (0), (1), (2), (3), (4), (5), (6), (7), (8),
  (9), (10), (11), (12), (13), (14), (15), (16),
  (17), (18), (19), (20), (21), (22), (23);

create temporary table today_t
select h, count(distinct ip) c 
         from ACCESS 
         where DAY(dtime) = DAY(curdate())
         group by h;

create temporary table yesterday_t
select h, count(distinct ip) c2 
         from ACCESS 
         WHERE DAY(dtime) = DAY(date_sub(curdate(), interval 1 day))
         group by h;

plot 
    all axislabels, very light blue bar, light red bar
  with 
    no sides
    baroverlap 1
    skinny
    comment left " Number of Visitors"
    title x "Hour of Day"
    legend bottom
select hours_t.h, c2 as "Yesterday", c as "Today"
  from (hours_t LEFT JOIN today_t 
     on hours_t.h = today_t.h) LEFT JOIN
       yesterday_t
         on hours_t.h = yesterday_t.h;