A trend line (or ‘line of best fit’) is often added to scatter plots to summarise and highlight the strength of the association between the 2 plotted measures, as shown in the graph above. This strength is clear by both the angle and shape of the line, and the spread of data points around the line. For example, a 45° line indicates a strong, linear correlation; a flat line indicates no association; and a U-shaped line indicates a curvilinear association.
Caution! Only include a trend line on a graph if its coordinates have been calculated using statistical methods that are appropriate for the kind of data you are presenting (eg simple linear regression for correlated data). This is a widespread scientific standard, and thus will be assumed by the reader. Lines that are drawn freehand or calculated using inappropriate methods can overstate or understate the association between the 2 plotted measures.
