Also, you see a green plus icon next to the data element when it is over a valid drop target.

Description of the illustration GUID-85F9FA40-4662-4C94-AB30-FEF44765839B-default.gif
Note:
If you are not sure where to drag and drop any data element, simply drag and drop the data element anywhere over the visualization, and not to a specific drop target. The data element is automatically added to the best drop target on the canvas.
After you drop data elements into visualization drop targets or when you move your cursor outside of the visualization, the drop targets disappear.