Starting with version 4.1.0 TeamTime introduced the ability to use JavaScript / HTML5 based charts using the ChartJS library rather than Adobe Flash which is being phased out.
This requires a modern browser to work though :-
To complicate things a little - Internet Explorer can run in 'compatibility mode' where it works as if it were an older version using the X-UA-Compatible meta tag.
SharePoint 2013 will cause even the most up to date version of Internet Explorer (IE) to behave as IE 10 and in SharePoint 2010 as IE 8 - though in rare cases this can be overridden.
When set to "Auto" mode TeamTime will use JavaScript charts for SharePoint 2013 and above, and Flash charts for SharePoint 2010 and earlier.
You can override this behavior if you wish by choosing Flash or JavaScript rather than Auto. You may wish to do this if, for example, you use SharePoint 2010 but only access it using the Chrome browser and not IE (so JavaScript charts will work).
Read more about the Administration > Settings page.
If your users don't have a browser that can support JavaScript based charts they will receive an error message - either change them to a modern browser or switch to Adobe Flash charts.