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How to create charts and graphs

Page history last edited by gerryc 15 years, 4 months ago

Charts and Graphs

 

Source: http://www.mindtools.com/pages/article/Charts_and_Diagrams.htm

 

Choosing the right format

"A picture is worth a thousand words." This is certainly true when you're presenting and explaining data. You can provide tables setting out the figures, and you can talk about numbers, percentages, and relationships forever. However, the chances are that your point will be lost if you rely on these alone. Put up a graph or a chart, and suddenly everything you're saying makes sense!

Graphs or charts help people understand data quickly. Whether you want to make a comparison, show a relationship, or highlight a trend, they help your audience "see" what you are talking about.

The trouble is there are so many different types of charts and graphs that it's difficult to know which one to choose. Click on the chart option in your spreadsheet program and you're presented with many styles. They all look smart, but which one is appropriate for the data you've collected?

Can you use a bar graph to show a trend? Is a line graph appropriate for sales data? When do you use a pie chart? The spreadsheet will chart anything you tell it to, whether the end result makes sense or not. It just takes its orders and executes them!

To figure out what orders to give, you need to have a good understanding of the mechanics of charts, graphs and diagrams. We'll show you the basics of the Line graph.

First we'll start with some basics.

X and Y Axes - Which is Which?

To create most charts or graphs, excluding pie charts, you typically use data that is plotted in two dimensions, as shown in Figure 1.

  • The horizontal dimension is the x-axis.
  • The vertical dimension is the y-axis.

Tip:
To remember which axis is which, think of the x-axis as going along the corridor and the y-axis as going up the stairs. The letter "a" comes before "u" in the alphabet just as "x" comes before "y".

When you come to plot data, the known value goes on the x-axis and the measured (or "unknown") value on the y-axis. For example, if you were to plot the measured average temperature for a number of months, you'd set up axes as shown in Figure 2:

The next issue you face is deciding what type of graph to use.

Line Graphs

One of the most common graphs you will encounter is a line graph. Line graphs simply use a line to connect the data points that you plot. They are most useful for showing trends, and for identifying whether two variables relate to (or "correlate with") one another.

Trend data:

  • How do sales vary from month to month?
  • How does engine performance change as its temperature increases?

Correlation:

  • On average, how much sleep do people get, based on their age?
  • Does the distance a child lives from school affect how frequently he or she is late?

You can only use line graphs when the variable plotted along the x-axis is continuous - for example, time, temperature or distance.

Note:
When the y-axis indicates a quantity or percent and the x-axis represents units of time, the line graph is often referred to as a time series graph.

Example:

ABC Enterprises' sales vary throughout the year. By plotting sales figures on a line graph, as shown in Figure 3 , it's easy to see the main fluctuations during the course of a year. Here, sales drop off during the summer months, and around New Year.

While some seasonal variation may be unavoidable in the line of business ABC Enterprises is in, it may be possible to boost cash flows during the low periods through marketing activity and special offers.

Line graphs can also depict multiple series. In this example you might have different trend lines for different product categories or store locations, as shown in Figure 4 below. It's easy to compare trends when they're represented on the same graph.

 

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