Time is an observed phenomenon, which we have used as a measure of how events are ordered from the past through to present and into the future.  Everything inevitably changes over time, but observing how things change and how often they change is what time is useful for.  By observing patterns from the past, people can predict and plan ahead for the future.

In this post in our ongoing series on Displaying data, I will be looking at the type of data visualising used for showing data changing over time.

Line Graphs & Area Graphs

Commonly used to show trends and changes in data over time, line graphs work by plotting points on a cartesian coordinates grid and connecting them up to form a line.

By following the first point to the last, you see the story of how values have developed over time: steep slopes indicate a sudden change in values and the direction of the slope (downwards or upwards) indicates whether the values have increased or decreased.  Reoccurring patterns in the lines, such as repeating spikes can indicate a trend in the data.

Area graphs work exactly in the same way for displaying data as line graphs do, but with the spacing below the line being filled in with a colour, pattern or texture.

Also area graphs have two additional variations when displaying multiple data series:

• Grouped Area Graphs: lines are all plotted from the same zero axis.
• Stacked Area Graphs: lines are plotted from the previous point.

Timelines

The main function of timelines is to communicate time-related information through graphical means; a visual way of showing a list of events in chronological order.  Usually this is to tell a story, present a view of history or make an analysis of a series of events.

Some timelines work on a scale, while others, like the one above, simply display events in a sequence.  If a timeline does work off a scale, then this allows readers to see how far apart in time events have occurred and how events are distributed over the selected time period.

Timeline can also work in conjunction with over forms of data visualisation to show how quantitive values related to the events have changed over time.

Gantt Charts

Used to display a project schedule through the use of bars that illustrate the start and finish dates of tasks within that project.  As well as being useful for planning Gantt charts can also be useful for displaying and comparing the different ranges of time periods (durations).

The bars in a Gantt chart can also be divided by colour (or lack of colour) to show the percentage of completion in a given task.

Streamgraphs

A visually attractive way of displaying data of large datasets, to give a generalised view of categorical data over time.  Streamgraphs are very similar to a stacked area graph, but have no fixed x-axis and instead use a varying baseline, which results in a flowing organic pattern.

The shifting baseline can also help minimise the change in slope for each individual layer.  This makes it easier to perceive the thickness of any of the layers across the chart.  The downside to having a varying baseline and no fixed x-axis is that accurately reading and comparing the thickness of each layer is difficult.

Condegram Spiral Plots

A visualisation method developed by Dr Mark Conde to display the changes in space weather over the months of year.

Time starts from the centre and spirals outwards with the data values being represented through a histogram.

Candlestick Charts

Commonly used as a visual aid for making decisions in stock, foreign exchange, commodity and options trading.  Candlestick charts resemble a combination of a box plot (but with different functions) and a line graph.

Each “candlestick” shows an interval in time and is coloured based on whether the market is “bullish” (the closing price is higher than it opened) or “bearish” (closing price lower than it opened).  Bullish candlesticks are usually coloured green or white and bearish ones are coloured red or black.

Below you can see the anatomy of a single candlestick.  The box in the centre is called the “real body” which uses a bar to show the range on the opening and closing price.  The little lines on the top and bottom are known as the shadows (or wicks), which represent the highest and lowest traded prices of a security during the time interval represented.

Open-high-low-close chart

Another financial chart, which functions in the same way as a candlestick chart, but with a different design.

Instead of using bars/boxes, Open-High-Low-Close charts (or OHLC charts for short) use simply the length of lines to communicate the price ranges over time.