Add legends to plots

[jpoeppel]

It would be nice if we could add legends to plots, especially for saving them. Questions that need to be answered for this:

• How to provide the required information? Use channel description, e.g. the key for timeplots? Ideally one might want to change this only in the plot, not in his program.
• Display always or only when saving?
• Where to place the legend? Ideally the user would have fine grained control over this!