Graphs An Introduction

What is a graph?  A graph is a visual representation of a relationship between, but not restricted to, two variables.  A graph generally takes the form of a one- or two- dimensional figure. Although, there are three- dimensional graphs available, they are usually considered too complex to understand easily.  A graph commonly consists of two axes called the x-axis (horizontal) and y-axis (vertical).  A graph can show Discrete or Continuous data types. Data types determines which graph to use.

Discrete or Continuous Discrete: Data that can be separated by some interval, for example: Recording the shoe sizes of a class – Bar graph Continuous: When data collected is continuous, for example: Recording temperature – Line graph

All graphs (except Pie Chart) have common elements:  Axis x and y (also z in 3D graphs)  Axis labels  A title  Scales or Increments  Can represent negative values X-axis Y – axis Origin – does not always have to start at 0,0 X-axis label Y – axis label Scale or Increments This is a graph -20

X-axis Y – axis Co-ordinates If we draw a vertical line from any point on the x-axis and a horizontal line from the y-axis A (3,15) B (-4,10) C (-5,-15) the point at which they meet gives us the co-ordinate Back

Types of Graphs Four basic types of graph or chart Bar Pie Scatter Line

See creating a line graph presentation Line Graphs A good method of showing the relationship between two variables Back

Bar Chart / Graph Used to compare values in a category or between categories. The graph shown here makes a visual comparison of the fat content of types of cheese

Bar Chart / Graph Can be useful to study trends over time

Multiple (or group) bar graphs compare relationships of closely related sets Bar Chart / Graph Back See creating a bar graph presentation

Pie Charts A pie chart is used to show how a part of something relates to the whole. This kind of graph is particularly suited to showing percentages Back See creating a pie chart presentation

Scatter Graph Drawing a scatter graph is similar to drawing a line graph in that co-ordinates are used to plot the points. There are usually more points to be plotted in a scatter graph and the points can be in groups, therefore it is not possible to draw a line through all of the points. Height (cm) Weight (kg) Line of best fit Back A scatter graph shows how much correlation there is between two variables that you suspect may be linked for instance height and weight as shown here: It is sometimes useful to draw a “line of best fit” to show the trend

Summary A good graph:  Accurately shows the data  Grabs the reader's attention  Has a title and labels  Is simple and uncluttered  Clearly shows any trends or differences in the data  Is visually accurate (i.e., if one data value is 25 and another 50, then 50 should appear to be twice the size of 25).