11 How to Make a Graph
Dependent and Independent Variables
A graph is a visual way to represent quantitative data that can quickly convey the relationship between two variables. One variable is controlled by the experimenter and is called the independent variable. The other variable changes based on change in the independent variable, it is called the dependent variable. The independent variable should be placed on the x-axis, while the dependent variable is on the y-axis. We refer to or title the graph with the y-axis variable vs the x-axis variable.
Example 11.1
Suppose you measured the solubility of a compound (how much dissolves per Liter of water) at different temperatures. Which variable is the independent variable and should be placed on the x-axis? Which is the proper way to refer to the graph, Solubility vs Temperature or Temperature vs Solubility
Answer: The independent variable it the one the experimenter controls, so it is the temperature in this example. Graphs should be referred to as y vx x, so this would be a graph of Solubility vs Temperature.
Best Practices for Creating Graphs
Now that you know which variable to put on which axis, you can create your graph. Using Excel or other computer program will make the process easier, as the axes will be automatically generated with constant intervals between gradations and your data will be placed on the graph for you. Here are some guidelines to follow when creating your graph:
- Use the “Scatter” plot so the data points are shown but there are no lines connecting the points.
- Adjust the x- and y-axis ranges so that the data points fill the entire graph region.
- Title the graph.
- Label each axis with the variable it represents and include units on the label.
- If you expect the graph to fit to a function add the appropriate trendline and display the equation and R-squared value in an open area on the graph.
- If you are displaying only one set of data, delete the Legend.
