[R] How to read several text files at once!

Dear R users, I am a beginner in R. I have 506 text files (data frame) in one folder namely DATA. The files are called A1 to A253 (253 files) and B1 to B253 (another 253 files). Each file has two columns; V1 (row number) and V2 (the value for each row name). Now I would like to add the values of

[R] How to open more windows to make more graphs at once!

Dear R users, I have a data frame (test) including five columns of upper (numeric), lower (numeric), observed (numeric), best_sim (numeric) and stname (factor with 80 levels, each level with different length). Now I would like to write a short program to draw one graph as follow for each level