Show Legend button in Graph should insert legend even if there's only 1 data set

jpollak[CodePlex]
Currently, the show legend button on the graph ribbon inserts a legend in the graph if there are 2 or more data sets. I think a legend should be inserted even if there is only 1 data set so the function of this button is more clear to the user and so a
graph with 1 data set has the option of including the legend.

Start at the graph ribbon's Show Legend button handling and trace how it decides whether to insert a legend. Verify the behavior with a graph containing one data set, while preserving the existing behavior for graphs with two or more data sets; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.