Shows you what age and gender your visitors are. Based on various services like Facebook, BlogCatalog and MyBlogLog.
Two possible reasons: 1. You don't have any visitors using one of the 3rd-party-services. Or Blog Demographics couldn't find the gender and age for them. Please log in those services and check if you've had any visitors. If you have, recheck if you've entered the right data in the settings-page! 1. Your theme might not call wp_footer(). To solve this add just before if it's not already there.
Not everyone who specifies their age specifies their gender, too.
They aren't all using this service. The scale of the service-chart is stretched to make it easier to see the services that aren't used as much. Look at the bottom of the chart to see the scale.