If you are designing a trend with scripting or drag/drop methods to assign pens, you need a way to clear the chart. This is not a trivial task as you need to disconnect, clear memory and then remove elements from the trend. Here is code to correctly clear out a trend chart:
<code cpp>
{
  int areaCount, i, j;
  dyn_string curves;

  getValue("TREND1", "numTrendAreas", areaCount);
  for(i=0; i<areaCount; i++)
  {
    getValue("TREND1", "curveNames", i, curves);
    for(j=1; j<=dynlen(curves); j++)
    {
      setValue("TREND1", "disconnectDirectly", curves[j]);
      setValue("TREND1", "curveRemoveData", curves[j], getCurrentTime());
      setValue("TREND1", "removeCurve", curves[j]);
    }
    setValue("TREND1", "removeTrendArea", i);
  }
  setValue("TREND1", "addTrendArea");                                    // ready for new pens
}
</code>