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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