Is it possible to insert a Chart under a Tab using a "script component"?
Yes, I have done this in the past using Google's Chart API. You'd need to write your own JavaScript to loop over the data you want to chart and then send it in an appropriate format to the Chart API.
See: http://code.google.com/apis/chart/
There is no way to display a Rollbase-native chart using JavaScript. Rather you need to drag and drop the chart component where you want it to appear on the page.
Hope this helps
Matt