code: @Override
      public IRenderingElement generate(IReaction reaction, RendererModel model) {
          if (!showAtomAtomMapping.getValue()) return null;
          ElementGroup elementGroup = new ElementGroup();
          Color mappingColor = atomAtomMappingLineColor.getValue();
          for (IMapping mapping : reaction.mappings()) {
              // XXX assume that there are only 2 endpoints!
              // XXX assume that the ChemObjects are actually IAtoms...
              IAtom endPointA = (IAtom) mapping.getChemObject(0);
              IAtom endPointB = (IAtom) mapping.getChemObject(1);
              Point2d pointA = endPointA.getPoint2d();
              Point2d pointB = endPointB.getPoint2d();
              elementGroup.add(new LineElement(pointA.x, pointA.y, pointB.x, pointB.y, getWidthForMappingLine(model),
                      mappingColor));
          }
          return elementGroup;
      }
docs: {@inheritDoc}