Here's an example data / chart spreadsheet:
http://temp.ancientpc.net/po/piechart.xls

Ideally what I want to do is force the pie chart to use the most recent /
lowest data series through some way of calculating the lowest non-empty cell
location, but I don't know if that's possible.

If not, I just want to be able to choose which data series to plot. Even
though at the moment there's 8 series available under Chart - Source Data,
it will only plot series 1 in the pie chart.