GraphicElement CreateLineGraphicElement( 
   ILayoutElementContainer elementContainer,
   Polyline polyline,
   CIMLineSymbol lineSymbol
)

//Create a simple 2D line graphic and apply an existing line style item as the symbology.

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Build 2d line geometry
  List<Coordinate2D> plCoords = new List<Coordinate2D>();
  plCoords.Add(new Coordinate2D(1, 8.5));
  plCoords.Add(new Coordinate2D(1.66, 9));
  plCoords.Add(new Coordinate2D(2.33, 8.1));
  plCoords.Add(new Coordinate2D(3, 8.5));
  Polyline linePl = PolylineBuilder.CreatePolyline(plCoords);

  //(optionally) Reference a line symbol in a style
  StyleProjectItem lnStylePrjItm = Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(item => item.Name == "ArcGIS 2D");
  SymbolStyleItem lnSymStyleItm = lnStylePrjItm.SearchSymbols(StyleItemType.LineSymbol, "Line with 2 Markers")[0];
  CIMLineSymbol lineSym = lnSymStyleItm.Symbol as CIMLineSymbol;
  lineSym.SetSize(20);

  //Set symbolology, create and add element to layout

  //An alternative simple symbol is also commented out below.  This would elminate the four 
  //optional lines of code above that reference a style.

  //CIMLineSymbol lineSym = SymbolFactory.Instance.ConstructLineSymbol(ColorFactory.Instance.BlueRGB, 4.0, SimpleLineStyle.Solid);  //Alternative simple symbol
  GraphicElement lineElm = LayoutElementFactory.Instance.CreateLineGraphicElement(layout, linePl, lineSym);
  lineElm.SetName("New Line");
});