You can turn it off in the graphs by going to Favorites>>Report Settings and unchecking the box : Show "None," "Unspecified" and "Typed/Bookmarked" data in report graphs.
I know what 'None' means, but it just look very bad in the report. I had to filter this value out everytime when I create dashboard. I wish there is a setting in the reporting interface that allows me to hide 'None' per report basis.