Mrs. Taxdahl’s Digitool Class
How To Do Bar Graphs
Introduction
A bar chart or bar graph is a chart with rectangular bars with lengths proportional to the values that they
represent. Bar charts are used for comparing two or more values that were taken over time or on different
conditions, usually on small data sets. The bars can be horizontally oriented (also called bar chart) or vertically
oriented (also called column chart). Sometimes a stretched graphic is used instead of a solid bar. It is a visual
display used to compare the amount or frequency of occurrence of different characteristics of data and it is used
to compare groups of data.
Rules for Graphing
All lines must be equal distance apart. All units equal.
When numbering and labeling, you must use the same movement between each line (for example: 2, 4, 6, 8)
Always label the line on the y (vertical) axis. The y axis is reserved for the responding or dependent variable.
The x axis or horizontal axis can be labeled between the lines. The x axis is reserved for the manipulated or
independent variable.
Your graph must have a title. The graph title gives an overview of the information being presented in the graph.
The title is given at the top of the graph.
Examples:
One Independent and One Dependent Variable
Simple Bar Graph
The above "Factory" is our independent or manipulated variable, since there is no unit of measurement for
factories and no 'order' to the factories, the independent variable is nominal. The dependent or responding
variable is scalar, measured in defects/1,000 cars. Since the scalar dependent variable has a natural zero point
(i.e., absolute or ratio), all of the bars are anchored to the horizontal axis, giving a common point of
measurement.
1. Horizontal Bar Graph
Bar graphs can be shown with the responding or dependent variable on the horizontal scale. This type of
bar graph is typically referred to as a horizontal bar graph. Otherwise the layout is similar to the vertical
bar graph. Note in the example above, that when you have well-defined zero point (ratio and absolute
values) and both positive and negative values, you can place your vertical (manipulated or independent
variable) axis at the zero value of the responding or dependent variable scale. The negative and positive
bars are clearly differentiated from each other both in terms of the direction they point and their color.
1. Range Bar Graph
Range bar graphs represents the responding or dependent variable as interval data. The bars
rather than starting at a common zero point, begin at first dependent variable value for that
particular bar. Just as with simple bar graphs, range bar graphs can be either horizontal or
vertical. Notice in the horizontal example above, a reference line is used to indicate a common key
dependent variable value.
1. Histogram
Histograms are similar to simple bar graphs except that each bar represents a range of
independent variable values rather than just a single value. What makes this different from a
regular bar graph is that each bar represents a summary of data rather than an independent
value. For this type of graph, the dependent variable is almost always a scalar scale representing
the count, or number, of how many of a sample fall within each range of the independent variable.
In the example above, the sample is all the females in the U.S. The independent variable is age,
which has been grouped into ranges of 5 years each. You should try and keep the ranges for each
bar uniform (5 years in this case), with the exception possibly being the first and/or last range.
Two (or more) Independent and One Dependent Variable
1. Grouped bar graph
The grouped bar graph is the same graph as seen above but we added a second independent
variable, year. The initial independent variable, factory, is nominal. The second independent
variable, year, can be treated as being either as ordinal or scalar. This is often the case
with larger units of time, such as weeks, months, and years. Since we have a second
independent variable, some sort of coding is needed to indicate which level (year) each bar
is. Though we could label each bar with text indicating the year, it is more efficient to use
color. We will need a legend to explain the color coding scheme. Note that all of the bars
for each level of factory are touching each other, indicating visually that they are grouped
together.
1. Composite bar graph
Another alternative for a bar graph with two independent variables is to have the bars stacked
rather than side-by-side. This arrangement is useful when the summation of all the levels of the
second independent variable is as or more important than the values for each level. In the upper
example, it is very easy to read the summed weight of all of the different materials in each sample.
There are, however, tradeoffs. The stacking of the bars means there is no common baseline for
the individual bar elements, making it hard to make direct comparisons for the subcategories. For
example, it is hard to compare the iron content of the three samples. A particularly powerful use
for the composite bar graph is when the sum of all the dependent variable values for each bar is
the same, such as when the values are a fraction of a whole. In the bottom example, the sum of the
three different types of fats will always equal 100 percent. With this layout it is easier to see the
relative portions, if not the absolute values, of a particular fat type across oils.
Specific tips for bar graphs on Excel:
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Vertical bar graphs are called column graphs in Excel
Horizontal bar graphs are called bar graphs in Excel
The clustered sub-type will do single bar and grouped bar
graphs
The stacked subtype will do composite bar graphs
Range bars cannot easily be done in Excel without additional
custom graph types loaded
Double-clicking on the bars will allow you to set your
preferences for bar display. Under the Options tab you can set
the ratio of the bar width to gap between bars