Given a set of points like this:
$ \{0.7, 1.0, 1.5, 2.0, 2.3, 2.6, 3.1, 3.6, 3.9, 4.2, 4.7, 5.2, 5.5 \} $
i want to creat a set of lists so that each of the lists keeps the numbers between a closed interval of length 1. for example my set of lists should be like this:  

$\{[0.7, 1.0, 1.5], [2.0, 2.3, 2.6], [3.1, 3.6, 3.9], [4.2, 4.7, 5.2], [5.5] \}$  

where first list keeps numbers between 0.7 to 1.7 , the second list keeps the numbers between 2 to 3 and third list keeps numbers between 3.1 to 4.1 and fourth list numbers between 4.2 to 5.2 and the fifth list the nembers between 5.5 to 6.5.  

my question is how can i find the Lowest possible number of intervals for a set so that the intervals are unit length and they contain all of the given points?