Why does the 'data' button disappear...
 

After creating a pivot chart, a button labeled 'Data' is created that shows
the column heading of the original data (before pivoting).

Multiple selections are available on that button, but when only one is
taken, the chart is built and the 'Data' button disappears. Only way to get
it back is to undo the pivot (which only occassionally works). The 'Data'
button stays visible if two or more selections are made.

What is causing this odd behavior?