Using this code to start building my chart:
      chartControl.Clear();
      chartControl.Size = new Size(options.width, options.height);
      chartControl.BackgroundStyle.FrameStyle.Visible = false;
      chartControl.BackgroundStyle.FillStyle = new NColorFillStyle(Color.Transparent);
      NCartesianChart chart = (NCartesianChart)chartControl.Charts[0];
If I have the line setting the fillstyle to transparent the old charts never clear. If I redraw the chart it just stacks on top of previous. If I remove the fillstyle line, it behaves correctly.