How Do I Plot Points on a Graph?
Plotting Geologic Data in x-y Space
Points on x-y Plots Can Also Be Called:
Ordered pairs
(Cartesian) coordinates
Why Should I Plot Points?
In the geosciences, we deal with large volumes of data, both observational and measured. This may be in the form of climate data, rock chemistry, elevation measurements, seismic data, etc. We generally compile data into tables, and when we want to know the relationship of one variable to another, one of the easiest ways to do that is to put that data on a plot. Take the table of data to the right. Just looking at it (you can click on the image to open a bigger version in a new window), can you tell what the general trend in CO2 values has been for the past fifty years? Has the trend changed in the past ten years? How about the last five years of record? Does the data vary from month to month? Are there seasonal cycles? So many questions! And they can all be answered with a simple x-y plot.
Show me an x-y plot that can answer some of these questions
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Where Is Graphing Used in the Geosciences?
Geoscientists use graphs to illustrate all kinds of issues in the science. In introductory geoscience courses, you may be asked to plot data in conjunction with units that deal with:
- rock compositions
- topographic maps
- streams and floods
- and almost any topic that might be covered in your course
If you are struggling to remember how to plot points, this page is for you! Below you will find some simple steps for plotting points on an x-y graph and links to pages to help you with the next steps.
Simple Rules for Plotting Points
Any plot or graph that has two axes is an x-y (or bivariate) plot. One axis (generally, the horizontal one) is the "x-axis" and the other (the vertical one) is considered the "y-axis". But you can use any variable for either one; all you need is a data set that has two sets of related data. Below there is an example of a set of data points for how basalt melting temperatures change deeper in the Earth.
Table showing how melting temperature of basalt changes with depth in the Earth. Modified from Tarbuck et al., 2008, Applications and Investigations in Earth Science, 6th edition. When you plot data on a graph, there are several steps that you can follow to make sure that you don't forget anything:
- Make sure that you have two variables to work with (two columns of data). In the case of the table above, the two variables are depth (km) and basalt melting temperature (C).
- Decide which of the variables is going to be represented on the x-axis and which will be on the y-axis. In some cases, you will be provided with a graph that has the axes labeled. A general rule (and one that many spreadsheet and graphing programs use) is that numbers in the first column of a table will be on the x-axis. However, geologists do not always follow this rule, so make sure you check.
- Label the axes on your plot and determine the appropriate scale (if the graph is not already labeled).
- Begin by plotting the first two pairs of numbers (the top row of numbers). In the case of the basalt melting temperatures, the first two numbers are (0, 1100). In other words, we are going to plot a point at x=0, y=1100. How do we decide where to put a point? Follow these simple steps:
- First, find the value for x on the x-axis. In the case of basalt melting temperatures, x = 0; so, find 0 on the x axis.
- Next, find the y-value. In this case, y = 1100, so find 1100 on the y-axis.
- Your point should be plotted at the intersection of x = 0 and y = 1100. (If you draw one line up vertically up from x = 0 and another line horizontally from y = 1100, where they cross is where you should put your point!
- Finally, plot the point on your graph at the appropriate spot.
- Continue to plot pairs of points from the table (in rows) until you have plotted all the points.
