html, body {

    width: 100%;

    height: 100%;

    margin: 0;

    padding: 0;

}

#container {

    position: absolute;

    width: 100%;

    top: 55px;

    bottom: 0;

}

fieldset {

    font-family: Verdana;

    font-size: 13px;

    border: none;

}

anychart.onDocumentReady(function() {

    // create chart

    chart = anychart.surface()

    // set data

    chart.data(generateData(parabaloid));

    // display chart

    chart.container('container').draw();

});

// generate a data set from function

var generateData = function(z_function) {

  var max_val = 5;

  var step = +document.getElementById('step').value;

  var output = [];

  for (var x = -max_val; x <= max_val; x += step) {

    for (var y = -max_val; y <= max_val; y += step) {

      output.push([x, y, z_function(x, y)]);

    }

  }

  return output;

};

// parabaloid function

var parabaloid = function(x, y) {

  return Math.pow(x, 2)+Math.pow(y,2);

};

// hyperbolic paraboloid function

var hyperbolicParaboloid = function(x, y) {

  return Math.pow(y,2)-Math.pow(x,2);

};

// plane function

var plane = function(x, y) {

  return x + y;

};