In order to delete a tag:
Open the Chart containing the tag you wish to delete
Click on the Other Options button (visible above the BodyGraph), then select Edit
Under Chart Tags click on the X next to the tag to delete it.
Repeat this step with all the Charts using the tag you want to delete. Once the tag has been removed from all of the Charts, it will be permanently deleted.
You can filter your chart library by tags under "Chart Attributes."
What to know how to add a Tag? Click here.