1) Sigma calculation had an error. The first data value in each bin didn't get squared.

2) "rrdfillmissing" was dummy. I have added code to do the work. The parameter has NOW to be in seconds. In doc it is
given in steps. The size of steps depends to much on size of graph so I think its easier to use seconds and internal
calculate the number of steps.

by Hans Jørgen Jakobsen

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@2155 a5681a0c-68f1-0310-ab6d-d61299d08faa