The spreadsheet containing the chart must be shared in Google Drive, either public or 'anyone with the link'. 

Note that when a chart is updated the results will not appear until the page is refreshed in the visitor's browser.

See Insert Charts by Steegle.

The problem with certain charts appearing blank has been fixed. If anyone still see this happen please post to the help forum the site URL and a hint to the viewer's email address (mostly the part after the @).

Bar Chart

Insert Charts