Scatter graphs are useful for showing cause and effect relationships between two variables or to examine the relationship of two variables with an unknown third factor. They help you look at a problem and identify the root causes and trends. A line graph helps you measure the change in the variables over a period of time. You should use a scatter graph if you are trying to find out if there is a connection between data, and use a line graph when you want to see how time effects the data.
Since scatter graphs help you determine the relationship -- or correlation -- between two different variables, usually a large amount of data is used. To create a scatter graph you need two sets of data. One is plotted along the horizontal -- or x axis -- and one is plotted along the vertical -- or y axis. If you were looking at the impact of a hazardous waste plants on children's health, the x axis could be for number of children with cancer in a neighborhood, while the y axis would represent the number of toxic waste facilities in that neighborhood. If there were nine children in a neighborhood who had cancer and that area had five hazardous waste sites, you would put a circle on the graph in line with the five on the side and the nine on the bottom axis. After plotting several neighborhoods you could determine if there is a correlation between the two variables.
After graphing your data you need to interpret it. Try to draw a line between the most clustered areas of the data points. If it is possible to make a straight line you can infer that there is a high correlation between your two variables. If the line goes from the lower left of your graph to the upper right -- lowest to highest -- then it shows a positive correlation. If the line goes from the highest point on the vertical axis -- upper left corner -- to the furthest point on the horizontal axis -- bottom right -- then it shows a negative correlation between the variables.
Line graphs help compare data that continually changes over a period of time. You could track your weight using a line graph. The horizontal axis would have the weeks or months of the year marked on it, while the vertical axis would be for your weight. Each time you weigh yourself you would mark the corresponding dot on the graph to match up the two axes. Draw a line between each dot -- like playing the child's game connect-the-dots.
Line graphs are easy to read. If the line goes up -- from lower left to upper right -- there is an increase, but if goes down -- from upper left to lower right -- there is a decrease. If you are trying to lose weight your graph will likely start towards the upper left corner and gradually move towards the lower right. Fluctuations are very apparent with a line graph. This allows you to track how certain events -- holidays, special occasions, illness -- affected your weight loss efforts.