horizontal key items

Hi,
the key in my plot has 5 elements. Using "set key", they are displayed
in the following way:

item1
item2
item3
item4
item5

If I use "set key horizontal", I obtain
item1   item2
item3   item4
item5

but, If I would like to obtain something like this:
item1 item2 item3 item4 item5

(that is, all the elements in one row)

how can I do?

Thanks in advance,
 M