Setting excel chart minimumscale using C#

I can't seem to figure out how to set the minimum scale of a excel chart using C#.

The xaxis for the chart is loaded with values from columns "A", which are hr:min values {"08:32","08:33",...}.

I want my axis labels to start at "09:00", and I figured that setting the minimumscale is the only way of doing this.

Xaxis.MinimumScale seems to want a double value, but I want to tell it to start at 09:00. My attempt is below, but it fails when trying to set the string to the double value.

Thanks

        Excel.Workbook oWB = (Excel.Workbook)oWS.Parent;
        Excel.Series oSeries;
        int length = numRows + 2;
        string colname;
        Excel.ChartObjects xlCharts;
        Excel.ChartObject myChart;
        Excel.Chart chartPage;
        Excel.SeriesCollection oSeriesCollection;开发者_StackOverflow社区

            ...

            xlCharts = (Excel.ChartObjects)oWS.ChartObjects(Type.Missing);
            myChart = (Excel.ChartObject)xlCharts.Add(10, 80, 300, 250);
            chartPage = myChart.Chart;
            oSeriesCollection = (Excel.SeriesCollection)chartPage.SeriesCollection();

            oSeries = oSeriesCollection.NewSeries();
            oSeries.Name = "Actual";
            oSeries.XValues = oWS.Range["A2:A" + length];
            oSeries.Values = oWS.Range["A2","A"+length];

            //Set the interval to 1 hour
            Excel.Axis Xaxis;
              //format the x-axis
            Xaxis = (Excel.Axis)chartPage.Axes(Excel.XlAxisType.xlCategory,
                          Excel.XlAxisGroup.xlPrimary);
            Xaxis.TickLabelSpacing = 60;
            Xaxis.TickLabels.Offset = 130;
            Xaxis.HasTitle = true;
            Xaxis.AxisTitle.Text = "Time of day";
            Xaxis.TickMarkSpacing = 60;
            Xaxis.MinorTickMark = Excel.XlTickMark.xlTickMarkNone;

            Excel.Range rg =(Excel.Range)oWS.Cells[28,1];
            object d = rg.Value2;

            Xaxis.MinimumScale = d;

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It may work if you convert the time (9:00) to a fraction (9/24). Excel 2003 used to accept axis scale parameters in time and date format, but 2007 only accepts this format in limited cases.