What is the Legend in salesforce.

By default, VisualForce charts include a legend, represented as a box that displays a list of labels
with their corresponding visual representation to serve as a key for the user viewing the chart.

The <apex:legend> component allows you to define attributes specific to the Legend, and
must be a child of the <apex:chart> container.

Series
The type of chart rendered is defined by its Series. The Series is a child element of the
<apex:chart> container, and is defined by specific VisualForce components and their associated
attributes:
• Bar Chart: <apex:barSeries>
• Line Chart: <apex:lineSeries>
• Area Chart: <apex:areaSeries>
• Scatter Chart: <apex:scatterSeries>
• Pie Chart: <apex:pieSeries>
• Gauge Chart: <apex:gaugeSeries>
• Radar Chart: <apex:radarSeries>