On your Piwik permission panel you must set anonymous can view for the specified website else data won;t be displayed according to the Piwik permissions. You may use a Piwik token (generated in your user panel at Piwik) if you want to avoid your data to be public by adding it to the url. IE :
http://mypiwik.com/index.php?token_auth=xxxxxxxxxxxx. __The token will be display in the HTML of the page therefor it is important you keep access to your stats only to trusted users.