Line chart - Omitting blank/zero cells in data range

Typically Excel 2000 line charts seem to plot any data point that has no Y value (i.e. empty cell) as zero. Even with the settings in Tools>Options>Charts:Plot empty cells as... set to 'Not plotted (leave gaps)' the problem remains.
Curiously however, when adding a second series to the line chart and plotting this on a secondary axis the problem (for this particular series) disappears.

All data in this instance is entered manually in to the data ranges (i.e. the problem isn't to do with formula results etc) but I understand that the issue also occurs when the plotted data is calculated and the result is "0", " ", or "N/A".

Has anyone got an explanation for this issue and/or had any comment from MS?

Thanks.