If you have loaded first tab and PieChart is located on the second tab that is hidden, then PieChart initially doesn't have a "space" as it's container probably has "display:none". When second tab is loaded and displayed you need to tell PieChart to update it's size by using API method:
If your project offers exporting PieChart image even if PieChart is not displayed on the current tab/page, then you can use following steps:
1. User requests export;
3. Actual export happens;
4. After export has happened, restore previous layout as it was before step 2.