In a chart or graph in spreadsheet programs such as Excel, the legend is most often located on the right hand side of the chart or graph and can sometimes be surrounded by a border.
The legend is also known as a Chart's Key.
To add to the confusion between legend and key, Microsoft refers to each individual element in a legend as a legend key.
A legend key is a single colored or patterned marker in the legend. To the right of each legend key is a name identifying the data represented by the key.
What Legend Keys Represent
Depending on the type of chart, the legend keys represent different groups of data in the accompanying worksheet.
For a pie chart or circle graph, each legend key represents only a portion of one data series.
In Excel, legend keys are linked to the data in the plot area so changing the color of a legend key will also change the color of the data in the plot area.