What are common mistakes students make when graphing data?
Well, the most common mistake in my experience is connecting the data points in a graph with a broken line touching ALL points. Instead you should try to fit the best line approximating the data points.
You MUST NOT do this:
The line in the second graph is called the "Best Fitting Line" and can be drawn using a simple ruler or, in complicated cases, using STATISTICS.
When you graph data you do it to "see" the phenomenon they represent (like to see a picture) and to try to predict the "future" trend of your phenomenon (by plotting a curve you can study).
The line in the first graph is very difficult to analyze whereas the second one, for example, is a straight line that can be easily written in mathematical form and studied.