Added some plot functions to extend existing series. 

Feel like the current plotting function lack the abilities to extend existing lines based on label/series name. So I added first function that extend current line base on series name. Also two helper functions that manipulate series in a chart. 

Not sure how they will be accepted, so I added them in a different branch. If they are acceptable, I can add more similar functions for other plots. 

Also this is a bug fix to previous pull request.