I guess I'm glad to hear it's not just me.

There's more to the wierdness. The default data series line thickness
in my Excel 2007 is 2.25 points. If edit one of the invisible series
and reduce this to 1.25 points, the series becomes visible. However,
in print preview it's gone, and printing the chart to a pdf file it's
gone. If I reduce it to 0.25 points, it does show up in print
preview, but not in a pdf file. That's what had me spending hours
looking for some chart formatting setting that was triggering this. I
finally just stumbled onto the thing with the zeros.

Are we having fun, yet?

Brian