Thanks for the help, guys. I have finally sussed it!

I recorded this macro while creating the chart:

    Range("C1:C721").Select
    Charts.Add
    ActiveChart.ChartType = xlLine
    ActiveChart.SetSourceData Source:=Sheets("Data").Range("A1:C721"),
+ PlotBy:= _
        xlColumns
    ActiveChart.SeriesCollection(1).XValues = "=Data!R2C1:R721C2"
    ActiveChart.Location Where:=xlLocationAsNewSheet
    ActiveChart.HasLegend = False
    ActiveChart.HasDataTable = False
    ActiveChart.Axes(xlCategory).Select
    With Selection.Border
        .Weight = xlHairline
        .LineStyle = xlAutomatic
    End With
    With Selection
        .MajorTickMark = xlNone
        .MinorTickMark = xlNone
        .TickLabelPosition = xlNextToAxis
    End With
    Selection.TickLabels.AutoScaleFont = True
    With Selection.TickLabels.Font
        .Name = "Arial"
        .FontStyle = "Regular"
        .Size = 8
        .Strikethrough = False
        .Superscript = False
        .Subscript = False
        .OutlineFont = False
        .Shadow = False
        .Underline = xlUnderlineStyleNone
        .ColorIndex = xlAutomatic
        .Background = xlAutomatic
    End With
    Selection.TickLabels.AutoScaleFont = True
    With Selection.TickLabels.Font
        .Name = "Arial"
        .FontStyle = "Regular"
        .Size = 5
        .Strikethrough = False
        .Superscript = False
        .Subscript = False
        .OutlineFont = False
        .Shadow = False
        .Underline = xlUnderlineStyleNone
        .ColorIndex = xlAutomatic
        .Background = xlAutomatic
    End With
[download]

And it converted to this code:

$sheet->Range("C1:C721")->Select;
my $chart = $workbook->Charts->Add;
$chart->{Name} = "Chart";
$chart->{ChartType} = xlLine;
$chart->SeriesCollection(1)->{XValues} = "=Data!R2C1:R721C2";
$chart->Location(xlLocationAsNewSheet);
$chart = $excel->ActiveChart;
$chart->{HasLegend} = "False;
$chart->Axes(xlCategory)->{MajorTickMark} = xlNone;
$chart->Axes(xlCategory)->{TickLabels}{Font}{Size} = 5;
[download]

See my home node for a picture of the chart and data, if you like. Ooh! and if anyone's looking for a meaty project, how about a script to automatically convert an Excel macro into perl? That's be nice! ;-)