iChart Trend does not refresh when NULL returned

I have a problem with an iChart displaying a trend. The chart receives data based on a selection from a dropdown box. Everything works fine as long as there is data, the trend updates correctly for the line selected from the dropdown. If, however, you select a line that is down, no current data available, then the iChart continues to display the trend from the last selection. These are based on a query that runs when a new selection is made and a button clicked.

When the page is first opened the iChart is blank until a selection is made. If the frame is refreshed, it refreshes the iChart to a blank state until a selection is made. What we need it to do is reset it to blank when no data is returned from the query, rather than caching the previous trend.

Any suggestions greatly appreciated.