Thank you for the opportunity to help you with your question!
The best way to show the above data is with a bar graph. It will show the differences in plant mass better than line or chart. It is given in a table format, and most Word or Excel programs will convert table format into graphs for you. Pie graph will not be useful because pie graph is good for percentages only. Line graph would work but would not show the full extent of the amount of change in plant mass. With such a large change from week to week, bar graph is the most graphic of the 4.
Please let me know if you need any clarification. I'm always happy to answer your questions.