Steps to Reproduce:
1. Make a new chart with some data.
2. Enter edit mode on the chart by double-clicking it.
3. Create a new trend line by clicking on Insert > Trend Lines...
4. In the resulting dialog, check the checkbox for "Show Equation".
5. Click "Ok" to close the dialog.
6. Double-click on the equation you just inserted.
The equation is editable, just like a text box (which it closely resembles).
The equation is not editable.
*** This bug has been marked as a duplicate of bug 77744 ***