A slight tweak to draw the pendulums too makes the simulation look much nicer:

      function draw (Ls, t, h) {
         // only for in the program, needed?
        for (var i = 0; i < Ls.length; i++) {
          ...
          ctx.lineTo(h * L, 0)
          ctx.arc(h * L, 0, 6, 0, 360)
          ctx.stroke()
          ctx.fill()
          ...
        }
      }

to do it right you really need to start another path, but this works fine in Chrome (looks little wonky in Firefox)