In this following sentence which one is right are or were
Samples (are or were) prepared from these four batches and strength is plotted in figure
If you are describing what was done in the past, say 'were'. If you are describing what is done every time an experiment is performed, say 'is'. You probably realize that your sentence is not complete.
I think you're trying to describe some sort of scientific procedure and the graphic results which appear in the article. Samples were prepared from these four batches; the strength of each is plotted in the figure (shown) [above / below]. CJ