The histogram

While a plotted curve assumes a continuous relationship between the variables by interpolating between individual data points, a histogram involves no such assumptions and is the most appropriate representation if the number of data points is too few to allow a trend line to be drawn. Histograms are also used to represent frequency distributions (p. 265), where the y-axis shows the number of times a particular value of x was obtained (e.g. Fig. 37.3). As in a plotted curve, the x-axis represents a continuous variable which can take any value within a given range, so the scale must be broken down into discrete classes and the scale marks on the x-axis should show either the mid-points (mid-values) of each class (Fig. 37.3), or the boundaries between the classes.

The columns are adjacent to each other in a histogram, in contrast to a bar chart (Fig. 37.4), where the columns are separate because the x-axis of a bar chart represents discrete values.