Edit in JSFiddle

var paper = Raphael(document.getElementById("canvas"));
var st   = paper.set();
var thickness = 30;

    var qBPath = function(ox, oy, dx, dy) {
       var curve = ((dx-ox) * 0.25)
       var path = "";
      path += "M "+ ox + "," + oy;
      path += "Q " + (ox+curve) + "," + oy + " " + ((ox+dx)/2) +","+ ((oy+dy)/2);
      path += "Q " + (dx-curve) + "," + dy + " " + dx + "," + dy;
      return paper.path(path);
    }
        
var downslope = qBPath(10, 10, 200, 200);

st.push(downslope);
st.attr({stroke : "#000", "stroke-width": 20})

<div id="canvas" style="width:200px;height:400px;padding:20px">