Should I use a scatter or a line chart?

Recently, during the Q&A portion of our Mekko Graphics webinar, I was asked how to know when to use a line chart versus when to use a scatter chart.   This is a great question, as scatter charts and line charts look very similar to each other, especially when a scatter chart is displayed with connecting lines.  But, there are specific uses for each, and there are easy questions to ask yourself to determine which chart will best portray the data you wish to present.

You will want to choose a scatter chart if you wish to show relationships among numeric values in several different series.  Each dot in a scatter chart represents a specific xy coordinate.  

 

Conversely, you might wish to use a line chart to show trends in data either at equal intervals or over a period of time.  Line charts present data against a common scale. The data is distributed evenly across the x axis.  If your x axis contains non-numeric values, such as months, it is generally better to use a line chart. For numeric x values, it is generally a better choice to use a scatter chart.

 
 

Other questions to consider are

1. The scale of the x axis. If, for example, you wish to use a logarithmic scale, you might choose a scatter chart as opposed to a line chart.
2. Do you wish to show patterns?  Scatter charts are most useful for illustrating patterns in data, like clusters of data, or outliers.
3. Do you wish to compare groups of data points without time as a factor?  Scatter charts show comparisons, and the more data you include in your scatter chart, the better the comparison.
4. Are you using an evenly spaced interval across the x axis?   Line charts are best when using evenly spaced numerical labels that represent a time interval, such as years.  On the other hand, a scatter chart always has two value axes to show one set of numerical data along a horizontal (value) axis and another set of numerical values along a vertical (value) axis. The chart displays points at the intersection of an x and y numerical value, combining these values into single data points. These data points may be distributed evenly or unevenly across the horizontal axis, depending on the data.
5. Does one series need to be plotted on a different scale? You can create 2Y Line charts, when one or more series are best displayed on a different scale.