Graphing
Presenting Scientific Data
Scientists accumulate vast amounts of data by observing events and
making measurements. After scientists organize data in tables, they
may display the data in a graph. A graph is a diagram that shows
the relationship of one variable to another. A graph makes
interpretation and analysis of data easier. There are three basic
types of graphs used in science - the line graph, the bar graph, and
the pie graph.
What is a graph?
Graphs are one of the most useful ways to organize and analyze
quantitative data. Graphs show patterns, communicate information,
and allow scientists to make predictions that can support or
disprove a hypothesis. Graphs help us see information better. When
we have a lot of information, graphs put all the information in one
place so that we can see it quickly and refer to it more easily. The
visual characteristics of a graph make trends in data easy to see.
When you use graphs, think carefully about the conclusions you can
draw from them. You want to make sure your conclusions are based
on accurate information and that you use scales that help make
your graph easy to read.
One of the most valuable uses for graphs is to show data collected
during an experiment. The graph shows your results.
Types of Graphs:
Line graphs
In laboratory experiments, you will usually be controlling one
variable and seeing how it affects another variable. A line graph is
used to show the relationship between two variables. The variables
being compared go on two axes of the graph. You can do a number
of things with the information in a line graph:
 You can determine how a variable changes over time,
 Given one variable, the other variable can be determined,
 You can show trends in the data, and
 You can make predictions about data that is not recorded
Setting Up a Line Graph, you must first identify the variables.
1. Determine the Independent Variable
The independent variable (aka the manipulated variable) is the one
you have control over or the thing you change. In a line graph, the
independent variable always goes on the horizontal axis, called the
x-axis..
2. Determine the Dependent Variable
The dependent variable (aka the responding variable) is the one you
don't have control over or the thing that changes when you change
the independent variable. In a line graph the dependent variable
always goes on the vertical axis, called the y-axis.
Question: Why are points connected on a line graph?
Answer: The points are connected because they are related.
Click on the graphic below to see an animation
about a velocity vs. time graph
Click on the graphic below to see an animation
about how to interpret line graphs
Example 1: an experiment has two variables, temperature and
plant growth. Temperature is the manipulated variable because you
chose the time intervals to take the measurements. The amount
of plant growth is the responding variable because its value depends
on what happens in the experiment.
Temperature Affects on
Tomato Plant Growth
Height of 30 Day Old
Plant (cm)
Temperature
(oC)
6
10
7
15
8
20
8.5
25
9
30
9.5
35
10
40
Example 2: an experiment has two variables, time and
temperature. Time is the manipulated variable because you chose
the time intervals to take the measurements. The temperature is
theresponding variable because its value depends on what happens
in the experiment.
Temperature of Water
Being Heated
Temperature
(oC)
Time
(min)
20
0
40
1
60
2
70
3
80
4
85
5
90
6
95
7
A person who has never seen your experiment can look at the
graphs above and know what took place.
Best Fit
When displaying results from experiments, it is sometimes
appropriate to draw a "line of best fit" - a line which is a better
indication of the overall trend in the data. The line of best fit may
take the form of a straight line or a curve. Results which are a long
way from the line of best fit (called anomalies) should be
ignored. You can see this in the example below. Click on this link to
see how to create a best fit line in MS Excel.
Scatter Graph
Some experiments or groups of data are best represented in a
graph that is similar to a line graph - called a scatter graph. As in a
line graph, the data points are plotted on the graph by using values
on an x-axis and a y-axis. Scatter graphs are often used to find
trends in data. Instead of connecting the data points with a line, a
trend can be represented by a best-fit line. A best-fit line is a line
that represents all of the data points without necessarily going
through all of them. To create a best-fit line in a scatter graph,
there should be approximately the same number of points plotted
on each side of the line.
If we connected all of the data points with lines, the lines would
create a zigzag pattern that would not tell us much about our data.
But if we find a best-fit line, we can see a trend more clearly.
Bar graphs
Bar graphs are similar to line graphs. They compare data that do
not continuously change. In a bar graph, vertical bars show the
relationships among data. Bar graphs make it easy to compare data
quickly. A bar graph is useful when you want to compare
information collected by counting, or similar data for several
individual items or events. Use a bar graph if you are not looking for
trends (or patterns) over time and the items (or categories) are not
parts of a whole.
A bar graph makes it easy to see how the data for one item
compares with the data for another. Bar graphs can also be used to
identify trends, especially trends among differing quantities.
To make a bar graph, set up the x-axis and y-axis as you did for the
line graph. The data is plotted by drawing vertical bars from the xaxis up to a point where the y-axis would meet the bar if it were
extended.
Click on the graphic below to see an animation about bar
graphs
An example of the type of data you might use for a bar graph could
be Leading Risk Factors that can cause death in Europe as seen
below.
A bar graph might be appropriate for comparing different trials or
different experimental groups. It also may be a good choice if your
independent variable is not numerical. (In Microsoft Excel, generate
bar graphs by choosing chart types "Column" or "Bar.")
Pie Graphs
Pie graph uses a circle divided into sections to display data. Each
section represents part of the whole. All the sections together equal
100 percent. Pie graphs (aka pie charts or circle graphs) are an
easy way to visualize how parts make up a whole. If you think of a
pie cut into pieces, you have a mental model of a pie graph. For
example you could use a pie graph if you need to compare different
parts of a whole, where there is no time involved and there are not
too many items (or categories).
Click on the graphic below to see
an animation about pie graphs
Click on the following links for more information about pie graphs:
What is a pie graph?
Drawing a pie graph
The figure below is a pie graph that describes the energy
consumption in the United States. The entire circle represents the
types of energy used in the United States. Each "slice" of the pie
represents a percentage of each type of energy being used.
How do I make a graph without using paper?
Constructing a graph using Microsoft Word
Constructing a graph using Microsoft Word 2007
Constructing a graph using Microsoft Excel
Web-based graphing tool (my favorite) - createagraph
Things to Remember when constructing a Line Graph
Step
1
2
What To Do
Identify the
variables
How To Do It
a. Independent Variable - (controlled by the
experimenter) - Goes on the X axis (horizontal)
b. Dependent Variable - (changes with the
independent variable) - Goes on the Y axis (vertical)
a. Subtract the lowest data value from the highest
Determine the data value.
variable range
b. Do each variable separately.
a. Determine a scale, (the numerical value for each
square), that best fits the range of each variable.
3
Determine the
scale
b. Spread the graph to use MOST of the available
of the graph space. Rectangular graph paper is turned so that the
variable with the widest range is drawn along the
widest side of the paper.
4
Number and
label
each axis
5
Plot the data
points
6
Draw the
graph
This tells what data the lines on your graph represent.
a. Plot each data value on the graph with a dot.
b. You can put the data number by the dot, if it does
not clutter your graph.
a. Draw a curve or a line that best fits the data points.
b. Most graphs of experimental data are not drawn as
"connect-the-dots".
a. Your title should clearly tell what the graph is
about.
7
Title the graph
b. If your graph has more than one set of data,
provide a "key" to identify the different lines.
Graphing Checklist
What Makes for a Good Graph?
For a Good
Graph, You
Should
Answer
"Yes" to
Every
Question
Have you selected the appropriate graph type for the data you
are displaying?
Yes / No
Does your graph have a title?
Yes / No
Have you placed the independent variable on the x-axis and the
dependent variable on the y-axis?
Yes / No
Have you labeled the axes correctly and specified the units of
measurement?
Yes / No
Does your graph have the proper scale (the appropriate high and
low values on the axes)?
Yes / No
Is your data plotted correctly and clearly?
Yes / No