title: How to add label to flutter chart?
body: <p>I try charts_flutter: ^0.6.0 dependencies, 
  the chart is worked very well.</p>
  
  but i want add label for x axis and y axis in chart.
  Label like rating and numbers of entries in this picture.
  <pre><code>charts.BarChart(
                _seriesData,
                animate: true,
                behaviors: [new charts.SeriesLegend()],
                animationDuration: Duration(seconds: 3),
              )
  </code></pre>