Types of graph

Different graphical forms may be used for different purposes, including:

• Plotted curves - used for data where the relationship between two variables can be represented as a continuum.
• Scatter diagrams - used to visualize the relationship between individual data values for two interdependent variables (e.g. Fig. 41.6) often as a preliminary part of a correlation analysis.
• Three-dimensional graphs show the inter-relationships of three variables, (e.g. Fig. 32.21).
• Histograms for frequency distributions of continuous variables (e.g. Fig. 37.3).
• Frequency polygons emphasize the form of a frequency distribution by joining the co-ordinates with straight lines, in contrast to a histogram. This is particularly useful when plotting two or more sets of data values on the same graph.
• Bar charts represent frequency distributions of a discrete qualitative or quantitative variable (e.g. Fig. 37.4).
• Pie charts illustrate portions of a whole (e.g. Fig. 37.5).

Fig. 37.4 Bar chart: showing the number of students belonging to each ABO blood group (n = 29).

Fig. 37.5 Pie chart: relative abundance of ABO blood groups in man.