I ran across this when generating a report for year over year sales. I kludged the query to generate the -24 month to -12 months (Last Year) column alongside the -12 month to today (This Year) column. Dual series chart works correctly. Then i repaired the query (which results in all null data for the -24 month to -12 month series, and the chart does not show either series, This Year, or Last Year, even though This Year has real numbers. Looking at the Visualize botton, it seems that all the configuration data is lost.
Then i re-kludged the query, thereby re-populating the Last Year data and the correct chart reappeard and the Visualize: config data was again evident.