viz.auto sometimes makes an empty picture

discovered while tinkering with the ppaml election stuff:
///fold:
var dist0 = {
  d: 0.47,
  r: 0.43,
  u: 0.10,
};

var alpha = 200;

var step = function(curDist) {
  var d = curDist.d, r = curDist.r, u = curDist.u;

  var joiners  = uniform(0, 0.2  ) * u;
  var leaversD = uniform(0, 0.001) * d;
  var leaversR = uniform(0, 0.001) * r;

  // assume that the deciders noisily mirror the people who have already decided
  var joinerSides = dirichlet(T.mul(Vector(_.values(_.omit(curDist, 'u'))), alpha)),
      joinersD = T.get(joinerSides, 0) * joiners,
      joinersR = T.get(joinerSides, 1) * joiners;

  return {
    d: d - leaversD + joinersD,
    r: r - leaversR + joinersR,
    u: u - joiners + leaversR + leaversD
  } 
}

var evolve = function(dists, steps) {
  if (steps == 0) {
    return dists
  } else {
    var curDist = _.last(dists),
        nextDist = step(curDist);
    return evolve(dists.concat(nextDist), steps - 1)
  }
}

var steps = 5;
///

var doPoll = function(pref, size) {
  _.object(_.keys(pref),
           multinomial({ps: _.values(pref), n: size}))
}

var KL = function(pProbs, qProbs) {
  return sum(map2(function(p, q) { p * (Math.log(p) - Math.log(q)); },
                  pProbs,
                  qProbs))
}

var pollsDist = Infer(
  {method: 'SMC', particles: 300},
  function() { 
    var path = evolve([dist0], 5);

    var poll0 = doPoll(path[0], 100);
    var poll3 = doPoll(path[3], 100);
    var goodness0 = -1 * KL(_.values(poll0), _.values(_.last(path)));
    var goodness3 = -1 * KL(_.values(poll3), _.values(_.last(path)));

    return {"rq": goodness3 - goodness0}
});

util.seedRNG(4);
viz.auto(pollsDist);
viz.table(pollsDist, {top: 10})
probmods / webppl-viz

viz.auto sometimes makes an empty picture #55
