first row of data in a graph not displayed

[SunWuKung]

I have a graph in a report. The graph doesn't display the first row of data.
If I open the Chart's Datasheet I can see that the data is there in the first row. Interestingly the first row is not numbered in the datasheet. If I drag all the data down one row the chart becomes correct, however next time I generate the chart the problem reappears.

I attached a picture of the datasheet when the first row is not displayed.

Does somebody know a solution to this problem.
Many thanks.
SWK