I would like to use two layers (units, calls) over the streets base map.

I have a timer loop that runs every 30 seconds and I would like to clear the units layer, then add the new unit markers to the unit layer.

      var units = new Graphic();  // graphic with units
      var calls = new Graphic();  // graphic with calls


      map = new Map({
        basemap: "streets-navigation-vector"
      });

      view = new MapView({
        container: "viewDiv",
        map: map,
        zoom: 12,
        center: {
          latitude: 38.8935,
          longitude: -77.0487
        }
      });


      var unitsLayer = new GraphicsLayer({
        graphics: [units]
      });

      var callsLayer = new GraphicsLayer({
        graphics: [calls]
      });


      map.add(unitsLayer);
      map.add(callsLayer);

 - Question 1) How do I clear a layer of markers without effecting the other layers?
 - Question 2) How do I add a marker to the units layer at run time?