A legend is displayed when the chart has multiple series. With the Chart Legend tag, you can customize the legend with a title, border and location on the chart along with other things. This tag must be nested in the Chart Display tag. 
<chart:display response="response" title="Current Products vs Recommended Products" stacking="normal">
    <series:facet name="current_products" type="column" title="Current Products"></series:facet>
    <series:facet name="recommended_products" type="column" title="Recommended Products"></series:facet>
    <chart:legend title="Legend Title"  align="right" vertical-align="top" layout="vertical" y="100" border-radius="5" border-width="1"></chart:legend>
</chart:display>
Chart Legend