原文：15 Questions All R Users Have About Plots
http://blog.datacamp.com/15-questions-about-r-plots/

原文太长了，这里只贴标题。

1. How To Draw An Empty R Plot?
2. How To Set The Axis Labels And Title Of The R Plots?
3. How To Add And Change The Spacing Of The Tick Marks Of Your R Plot
4. How To Create Two Different X- or Y-axes
5. How To Add Or Change The R Plot’s Legend?
6. How To Draw A Grid In Your R Plot?
7. How To Draw A Plot With A PNG As Background?
8. How To Adjust The Size Of Points In An R Plot?
9. How To Fit A Smooth Curve To Your R Data
10. How To Add Error Bars In An R Plot
11. How To Save A Plot As An Image On Disc
12. How To Plot Two R Plots Next To Each Other?
13. How To Plot Multiple Lines Or Points?
14. How To Fix The Aspect Ratio For Your R Plots
15. What Is The Function Of hjust And vjust Inggplot2?