Microsoft access chart

Hi there,
i have created a chart in a form using the chart wizard in access, it has a parameterized Query source. This query returns two data fields from my database. The chosen chart type is an XY scatter. The problem is that if the query returns no results(i.e. returns empty row), the second time the chart is opened with new parameters for the query, the values supposed to be on the y axis are now for the x axis (in the datsheet their column's title is x)and the ones supposed to be for the x axes are not shown anymore on the graph (on the datasheet their column is with no title)..i.e i'm having an empty chart..
Please try to answer my question..Thanks in advance..
Joelle.