I am new with plotly and I have couple of questions:
Can the plotly web server be deployed on premises? is there any guideline or procedure for doing that?
Can the plotly for R library (https://plot.ly/r/) be used for free even though one does not use the plotly web service?
Thank you
Yes and yes 
By default, the R library is completely free and open source (released under MIT license), but you may opt to premium services, such as sending a figure to an on-premise server via api_create().
You can review our On-Premise server options at https://plot.ly/products/on-premise/ and our hosted cloud options at https://plot.ly/products/cloud/. You can also purchase R developer support at https://support.plot.ly/libraries/r
Feel free to contact us with questions about on-premise options: accounts at plot.ly