title: Extjs chart loading mask
body: <p>Is there a way to mask chart while loading?
  LineChart has event 'afterrender', store has 'load' but it is not what i need.
  I see sufficient delay between them and final chart rendering.
  tnx</p>
  
  <h1>Env: extjs 3.3.1. flash</h1>
  
  <pre><code>  var chart = new Ext.chart.LineChart({
                      id: 'chart1',
                      store: store,
                      xField: 'name',
                      yField: 'visits',
                    listeners: {
                      'afterrender': function() {
                        Ext.getCmp('chart1').getEl().unmask();
                      }
                  }
              });
  </code></pre>