It is very important to pay attention to the scales that are marked on the graph. Otherwise, you will probably misinterpret the data. Here are some common issues to watch out for.
Graphs that don't start at zero
In this graph, it appears that the airplane is going to run into the ground. However, the vertical axis doesn't start at zero feet! In fact, the airplane is only decreasing it's altitude from 30000 ft to 15000 ft.
Crazy units
This graph looks just like the previous one, but the units on the altitude are inches instead of feet. Therefore the plane drops in altitude 15000 in or 1250 ft instead of 15000 ft.
Scales that increase in the wrong direction
This graph looks like the opposite of the first one, but again by paying attention to the scales, it isn't all that different. The direction that the scale increases in can be changed so that it best represents the data. Increasing is not always up and decreasing is not always down.
Sometimes the scales on a graph are unusual, like log y or 1/x rather than just y or x. This is done so that the data being plotted will come out to be a line, rather than some curved shape. When fitting a line to data, it is much easier to fit a line than a curve.
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