Graphing Basics
Graphing, Standard Curves, Unknowns
An important method in any science is the use of graphing to find the relationship (if one exists)
between two variable parameters. Graphs are tools we use to aid our understanding of the
relationships among variables. Plotting a graph provides us with:
• a visual image of the data and their relationships
• the ability to predict the results of a change in one variable if the other is held constant
In some cases, there is a direct, linear relationship between the parameters. The data obtained in these
experiments can be “fit” to produce a straight line on a graph described by the equation y = mx + b
where m = the slope and b = the y intercept.
Graphing Basics
Which axis is which?
The horizontal axis is the independent variable, like time; the vertical axis is the dependent variable,
that is, the one that changes in response to the independent variable, like hunger. Hunger increases as
a function of time, but time does not increase as a function of hunger. Time would increase regardless
of how hungry you feel, so time is the independent, and hunger is the dependent variable. Another
way to think about this is that the independent variable is the one you control and vary, plotted on the
horizontal axis, and the dependent variable (the one you observe or measure) is on the vertical axis. In
most scientific graphs, the data points themselves are visible, even if a line or curve is drawn through
them - this allows the viewer to see the underlying data and draw their own conclusions.
Label the Axes
Label all axes including units, for example: ie "Temperature (°C)" or "Volume (mL)". Use even
divisions and whole numbers for your axes. For example, do not use 2.3, 3.1, and 3.6 for your axis
divisions; instead, use 2.0, 3.0, and 4.0.
Add a Title
Add a title to the graph. Give your plot a good title that indicates the purpose of the graph, or what the
graph tells you. Don’t make your title simply the x and y axis labels; a reader can see that already.
What scale to use?
That is, what numbers should be on each axis? Should the axis begin at 0? How far out should the
numbers go? If you are plotting your college GPA as a function of time, the vertical axis does not
need to go above 4, nor below zero into negative territory. The horizontal axis, in years, may need to
go to 5, or 6, or 7 …… Make sure that you choose a scale that will include all the data points, but
don’t choose such a large scale that the curve takes up only a small part of the available graph. Choose
scales so that the curve covers the majority of space on the graph.
Figure 1: These three graphs show the
same data, but use different scales
along the axes. The top graph shows
the data most clearly. Note that each
graph contains the title and the axes
are labeled.
Plot the data points. Then what?
There are different procedures, depending on what it is that you are plotting. If you are plotting your
college GPA versus time, then there is no reason to expect that it will be a straight line. We do not
expect a linear correlation of the data. In this case, it would be perfectly reasonable to connect the
data points. However, if the graph is linear according to theory, draw the best straight line through the
points; that means that there will be as many data points above the line as below the line. The line is
the best average. For a linear plot y = mx + b, where m is the slope and b is the y-intercept.
The slope (m) is the change in y with respect to the change in x: (sometimes called “rise over run”). yintercept (b) is the value of y when x = 0. Is there a linear correlation in the data on the following plot?
Graph 1. Student Anxiety
5000
Student Anxiety,
4000
in SAUs
3000
2000
1000
0
0
1
2
3
4
5
Student time in college, years
FAQ: How to make scientific graphs
Can I just draw my graph on graph paper? I have a ruler!
Not in this class. You should start learning how to use Microsoft Excel or another graphing program
immediately. I will be available during office hours to help you learn to use a graphing program but the
best way to learn is to try. And then practice.
What variable goes on each axis?
By convention, the dependent variable is placed on the vertical axis and the independent variable is
placed on the horizontal axis of the graph. If you don’t know which variable is which ask yourself.
Which variable did I control or manipulate? This is the independent variable. Which variable did I
measure? This is the dependent variable.
Should I use a line graph or a scatter plot?
Most scientists prefer scatter plot graphs (in which each data point is plotted without any lines or
curves drawn) because it allows them to see the underlying data and draw their own conclusions. Bar
graphs are almost never appropriate in chemistry laboratory work. In this course, every graph must be
a scatter plot graph, or, at the very least, show each data point.
What information needs to labeled on each axis?
Each axis should have the variable shown as well as the units in parentheses, e.g., "Temperature (K)"
How do I make a great title?
Every graph should have a title, and this title must be more descriptive than just "Variable A vs.
Variable B." (The variables you are graphing are obvious to anyone who looks at your axes.) Ideally,
the title of the graph refers to and describes the experiment being done.
Should I draw a straight line through the data?
This is a complicated question. If you suspect, for theoretical reasons, that your two variables have a
linear relationship, then you may test this idea by using a computer program such as Microsoft Excel to
"fit" a line to your data. The computer draws a line through the data that is as close as possible to all
the points in the data. In the past, scientists used rulers to guess at such lines, but this practice is no
longer acceptable.
I think my data is linear, but I can't tell if the straight line is a good fit or not.
This is a common question, and in fact this is exactly the kind of question another scientist would ask
about your straight line fit. To answer it, you report (using the computer) the R2 value of the line; the
closer your R2 is to 1.00, the better the fit.
How do I know the equation of the straight line I drew through my data?
Have the computer report this value for you. It will be displayed in the form y = mx + b.
If I suspect my data has a non-linear relationship, should I draw such a line through it?
Yes. Many physical and chemical variables are related through a "square law" relationship, in which
one variable equals the square of the other (i.e. y = mx2). You can have the computer "fit" this kind of
curve to your data.
Do I need to include the raw data for the graph?
Yes. You should always turn in the data table that you used to generate your graph.
Creating a Graph Using Excel (MAC)
1. Open an Excel spreadsheet and enter the numbers you want to plot in columns A and B.
2. Highlight columns A, B and C
3. Click Insert, Chart, XY scatter
4. Label the X and Y-axis and add a title using the toolbar.
5. Label the series. Right click on the legend (little box on the graph). Click on “select data” and give
each data series a correct name
Creating a Graph Using Excel (PC)
1. Open an Excel spreadsheet and enter the numbers you want to plot in columns A and B.
2. Highlight columns A, B and C.
3. Click Insert, Scatter and select the plot that has points only (no lines)
4. Label each axis and give the graph a title using the layout tab.
5. Label the series. To give the series a correct name in the legend click on the chart to open “chart
tools” and click the “design” tab. In the “data” box select “select data”. In the box that pops up
highlight the data series you want to rename and then click “edit” and enter the series name.