Vertical bar graphs

Vertical bar graphs

Vertical bars may be used for time-series data when specific data values need to be emphasised. It can be difficult for readers to identify values for a specific time point on line graphs, which instead highlight trends or directional changes in data values over time.

Caution! Data across a large number of continuous time points should be displayed on a line graph. Using a line graph for few data points can suggest trends in the data that cannot confidently be inferred.

Visualising data values over time, for small time series

Vertical bars are useful for comparing data values across a relatively small number of time points (around 8 or less). Using vertical bars spaced along a horizontal (x) axis of time points draws on readers’ intuition to think about time horizontally and linearly, from left to right:

Visualising differences from a baseline

Displaying deviations in data values from a baseline (eg the first year of measurement or the year before a new environmental management system was introduced) can be a clear and powerful way to communicate changes in a measure over time or across groups.

In (vertical) deviation bar graphs, data values are shown as differences from a meaningful baseline, rather than raw values of measurement. As with all bar graphs, bars should start at 0 on the y axis. However, the horizontal x axis no longer intersects at the base of the y axis – the value of 0 may be positioned some way up the y axis to accommodate negative deviations from the baseline.

Bars (ie data values) above the x axis indicate positive differences from the baseline; bars below the x axis indicate negative differences from the baseline.

Data values can be absolute deviations from a baseline or percentage changes from the baseline.

Histograms: visualising frequency distributions

Data that describe the frequency or ‘counts’ of a measure across each of its possible values or intervals are typically presented as a vertical bar chart known as a histogram – for example, the number of days per year with temperatures across the range of –10 °C to 50 °C.

Vertical bars are used instead of horizontal bars for this type of data because the intervals along the horizontal axis are related. That is, they are the range of a single measure rather than the discrete groups or categories described for horizontal bar graphs. Like time-series data, readers tend to understand measures that are displayed on a left-to-right plane as part of the same sequence:

Visualising frequency distributions of text scores for students using a histogram.

As for time-series data, vertical bar graphs are recommended for displaying frequency distributions for measures with relatively few intervals, or when the reader needs to identify individual values. For measures with a large number of intervals, and where authors want to convey the shape of a distribution, a line graph known as a frequency polygon is recommended.

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