Separate points from databases can be displayed as point layers only.
To display them as lines, you have to add the points from the DataTable to a line shape, then add the shape to the map.
AspMap.Points line = new AspMap.Points(ShapeType.Line);
foreach (DataRow row in dataTable.Rows)
MapShapes ms = map.MapShapes.Add(line);
ms.Symbol.LineColor = Color.Green;
ms.Symbol.Size = 2;
The Map.MapShapes collection is persisted between postbacks.