[R] how to make legends on histograms

[Andreas Zankl]

I have a histogram with histograms for several datasets superimposed. 
How can I add a legend that indicates which dataset uses which 
linetype?

Thanks
Andreas

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Dr. med. Andreas Zankl
Division de Pediatrie Moleculaire
Clinique Infantile 02/50
CHUV
Avenue Pierre Decker 2
CH-1011 Lausanne
Suisse
Tel.: +41-21-3143778
Fax: +41-21-3143546
Email: andreas.zankl at hospvd.ch