PlotBy Property [Excel 2003 VBA Language Reference]
Returns or sets the way columns or rows are used as data series on the chart. Can be one of the following XlRowCol constants: xlColumns or xlRows. Read/write Long. For PivotChart reports, this property is read-only and always returns xlColumns.
This example causes the embedded chart to plot data by columns.
Worksheets(1).ChartObjects(1).Chart.PlotBy = xlColumns