All workbooks that are saved to the database are saved to a file called EUL5_DOCUMENTS where DOC_NAME is the name of the workbook(s). The code for the workbook will be contained in a LONG RAW field called DOC_DOCUMENT.
Because it is a LONG datatype you cannot use SQL to query it directly. However, you should be able to create a function that will allow you to convert the LONG into text and then you should be able to search for the word CHART.
Not saying 100% that this will work but it might be worth a shot.
For help on creating such a function I'd like to refer you here: http://www.dba-oracle.com/t_select_long_table_column_sql.htm