What is the definition of rock strata and law of original horizontality?

It's called the Principle of Original Horizontality
 it just means what it sounds like: that all rock layers were originally horizontal.
Of course, it only applies to sedimentary rocks.
Recall that sedimentary rock is composed of  sediments, which are deposited and compacted in one place over time.

No because a circle or pie graph usually shows percentages you'd need a line graph