Graphing: Line Graphs

Predetermined Sequences

But, why not the column graph? Does it not show the data? Does it not show an increasing trend? Does it not deserve my love? No. The rule of thumb is that if the values on the x-axis have a predetermined order, we use a line graph. The values on the x-axis are all related to each other in a predetermined way. This requires us to use a line graph, connecting all the x-axis values to show their relationship. Temperature is a great example of this. Could we have reordered the x-axis into any order we wanted? I mean, we could, but it would then make no sense. Temperature, time, age, concentrations, pHA measure of hydrogen ion concentration in a solution., and a whole bunch of other biologically-related items have predetermined relationships and will be graphed with a line.

Common Problem

When using Excel, a common problem occurs where Excel will mistake your independent data for a dependent variable. This results in an extra line on the graph when you only expected one. Below is a graph that shows the number of fish species observed in the years after establishing a preserve. Well, the blue line shows that anyway! The orange line seems to represent a data set that has values exactly the same as the data set for the independent variable. Because the years are in numeral form, Excel thought that is was a dependent variable. Be smarter than Excel and be aware when and if this happens. This can be solved by exploring the Data Source options of the graph.

Graphing Rates

Graphing rates can be confusing to students because the x-axis is not the independent variable. For example, we might want to calculate the rate at which fungus spreads on a piece of bread when held at different temperatures. Four slices of bread were inoculated with fungus by wiping the bread on the floor (fungal spores are literally everywhere). Each slice of bread was held in a different temperature. For 10 days total, the surface area of the growing fungal colony was measured in some vague and random unit. Here are the results:

Notice how time is on the x-axis. But, the time applied to all bread slices was the same and is thus a controlled variable. The lines on the graph represent the different temperatures in which the bread slices were held; our real independent variable. Wait…or, does it? That graph up there has two numbers listed and two locations listed. Which is it, temperature or location? Temperature. Final answer. The locations are just the tools used to create that temperature. We are not investigating how quickly fungus engulfs a piece of bread in my living room. No one cares about my living room. We are investigating how quickly fungus engulfs a piece of bread at room temperature, created by my living room.

The series legend on this graph (at the very bottom with the colors and temperatures) correctly uses the degree symbol. Try your best to use graphing software that can use these types of font settings. Sometimes, you can copy and paste things such as symbols or superscript/subscript fonts into a text box and add it that way.

Multiple Line Graph

Multiple line graphs are a great way to compare trends within the treatments of an independent variable. The graph below shows the variation, by month (a controlled variable) of young-of-the-year (YOY) striped bass at certain locations along the Hudson River. The locations are the independent variable, each deserving of its own line on the graph. The student started out by graphing with the 3 site locations on the x-axis. This produced a graph looking like this:

I do not find this depiction of the data easy to decipher. Graphing the data in this way doesn’t help us compare these data sets. Once again, decisions before (or lack thereof) opening Excel are crucial in determining the graph. What the student has done here is reversed their rows and columns of data. Luckily, Excel has an option where you can tell it that you want to reverse rows and columns, instead of forcing you to retype all your data.

The Smoothed Line

There is a subtle element on this graph that makes it more visually appealing: a smoothed line. Most graphing software have the option to employ a smoothed line instead of a harshly angled line. You also have the ability to choose the dots on the line or the data points. I chose circles, but I could have chosen dodecahedrons, if I wanted to. I can even choose different shapes for the different lines. But, maybe that is too busy and will detract from what I’m trying to show the reader in my graph. Maybe. You can never know until you try it and see!

Graphing can be a process of trial and error. Now is the time to trial and error.