You are encouraged to work together on the homework and learn from each other. Discussion on how to approach a problem, both in interpretation and derivation of the solution, is an important part of the learning process. Each of you must individually produce the final solution however. It is very important that you actually do the work yourself. Simply copying homework will do you no good, and if you are caught in it, the proverbial book will be thrown at you.
Plagiarism cuts at the very heart of the academic world.
Graphing is perhaps the best tool for grasping relationships between variables in physical problems. You will find that is particularly true of diffusion processes. As a rule of thumb, if your calculator costs more than a date at the movies, I will probably confiscate it for the duration of the final and midterm. This shouldn't be too much of a problem, the exams are written to test your understanding, not your data entry. Certain questions will be marked as assessed questions, for every sheet.