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Graphing
Descriptive Studies on Two Y-Axes Graphs
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It is easy to misrepresent information simply by changing the axes on
the graph. Graphs with 2-Y axes and 1-X axis are
particularly challenging.
Here are some basic guidelines to be sure that your
graphs are showing undistorted correlations.
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Start the axes at zero
In your first attempt at developing a graph, you should always
anchor the Y axis at zero. Then you can
modify the minimum value if doing so does not change the relationships shown by the
graph. If you are not starting an axis at zero, you
should explicitly state why you have chosen an
alternative. Here are two common reasons for not starting
a graph at zero.
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When the lowest values in the data are far from
zero, you may choose to start at a higher value to save space. Note
that this may exaggerate the size of any differences.
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When zero is not a
biologically plausible value for the data being graphed, set the
minimum values just below the lowest values in the data. For
example, serum sodium levels are always far above zero.
Set the axes at logical
levels
When you are analyzing data, set the minimum and maximum values
on the axes at logical levels. To
determine the range of values that need to be displayed on the
graph, identify the largest value to be graphed
and round up to a slightly larger number. For example, if the
largest integer is 778, it might be logical to round to 800 as the
largest value on the axis. As long as the axes
emphasize a valid relationship, it is appropriate to
modify the minimum values. However, it is important to
indicate you have done this by putting a short gap between 0 and the
minimum number you chose so the reader is visually aware of how you
made the graph. Another alternative is to provide a full-set graph anchored at
zero and a sub-set graph that contains a break in the axis to provide the reader with all the information used to
develop the modified graph. Establish an appropriate
interval size
Select an interval size that does not crowd the data together
and prevent the relationship from being visible. Often, increments of
10, 100, 1000, etc. are used as standardized intervals.
Both Y axes must be comparable
When you use a2-Y axes graph, the range on both Y axis needs to
be similar. In other words, if the scale on the left Y axis is
0-100, the scale on the right Y axis should be in an equivalent range.
Use the same scale if several graphs will be
compared
To provide accurate analysis and evaluation of multiple
graphs, you might set the same scale for the sake of
comparison. It is important to be consistently clear what
scales and legends are being used and what they mean.
A Two Y-axes Graph in Practice
This
graph was also cited by Kaplan and Wein based on reinterpretation of data
presented by Foege, Millar, and Henderson. (3,4)
Question
#4
DIRECTIONS:
Complete the following based upon the instructions
provided within each question and provide answers based upon the
relationships that appear.
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