Line graphs compare two variables. Each variable is plotted along an axis . A line graph has a vertical axis and a horizontal axis. So, for example, if you wanted to graph the height of a ball after you have thrown it, you could put time along the horizontal, or x-axis, and height along the vertical, or y-axis.
As I mentioned before, each type of graph has characteristics that make it useful in certain situations. Some of the strengths of line graphs are that:
They are good at showing specific values of data, meaning that given one variable the other can easily be determined.
They show trends in data clearly, meaning that they visibly show how one variable is affected by the other as it increases or decreases.
They enable the viewer to make predictions about the results of data not yet recorded.
Unfortunately, it is possible to alter the way a line graph appears to make data look a certain way. This is done by either not using consistent scales on the axes, meaning that the value in between each point along the axis may not be the same, or when comparing two graphs using different scales for each. It is important that we all be aware of how graphs can be made to look a certain way, when that might not be the way the data really is.
Let's take a look at an example.
In a few years, you might be interested in getting some kind of parttime job. You find the following line graph, which plots the minimum wage versus time from Ocober, 1938, to September, 1997. What kinds of things might you be able to tell from it?
Answer the following questions by looking at the graph, and then click on Answers to compare your responses.
1. What was the minimum wage in January, 1978?
2. When did the minimum wage reach $3.35?
3. Between what time periods was the largest increase in minimum wage?
4. Based on your observations of the graph, make a prediction about what the wage might be in the year 2000.
5. What about the scales used on the graph might make the data appear differently than how it really is?
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