Legend implementation

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Use the library and want to add a legend

What is the expected output? What do you see instead?
Legend appearing in the plot

What version of the product are you using? On what operating system?
git checkout

Please provide any additional information below.
This is the most awesome project i've seen in a very long time! A plotting 
library this easy to use is long overdue in C++. THANK you for your awesome 
work, and view this as a feature request and a sign that your efforts with this 
library is MUCH appreciated!

Original issue reported on code.google.com by [email protected] on 29 Dec 2011 at 1:24