When adding values to a serie by adding the whole array at once, the property "Last" of xValues/yValues of the series still remains 0. This leads to errors using the DownSampling-Function. Example: dim xVal(100) as double, yVal(100) as single xVal(0) = 1 yVal(0) = 1 .... tChart1.series(0).add(xVal, ...