title: Set Y-axis grid density in Rickshaw.js
body: <p><img src="https://i.stack.imgur.com/m4R9T.png" alt="enter image description here"></p>
  
  <p>I have a graph detailing wind speeds where I would like to have the grid on the Y axis have ticks every 5 (m/s).
  <strong>How do I set the density of the Y axis grid?</strong></p>
  
  <pre><code>// triggered when data has been loaded via ajax
  $graph.on('graph.render', function(e, d) {
      var graph, y_axis;
  
      // Fixtures
      palette = new Rickshaw.Color.Palette();
  
      graph = new Rickshaw.Graph( {
          element: $graph.find('.chart');
          width: $graph.innerWidth() - 20,
          height: $graph.innerHeight() - 20,
          series: [
              {
                  key: 'min',
                  name: 'Min Wind Speed',
                  color: palette.color(),
                  data: d[0].data
              } // ...
          ]
      });
  
      x_axis = new Rickshaw.Graph.Axis.Time({
          graph: graph,
          timeUnit: {
              seconds: 600,
              formatter: function(d) {
                  return d.toUTCString().match(/(\d+:\d+):/)[1];
              }
          }
      });
  
      y_axis = new Rickshaw.Graph.Axis.Y( {
          graph: graph,
          orientation: 'left',
          element: $graph.find('.y-axis')
      });
  });
  </code></pre>