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.
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:
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.
