|InfoVis.net>Magazine>message nº 191||Published 2007-12-31|
|También disponible en Español|
The digital magazine of InfoVis.net
Every respectable map or graphic representation includes a legend, a text or graphic that explains the meaning of the conventions concerning the symbols or visual variables, scales, etc. For example blue for rivers, a small castle to denote a monument, or a gradation of colours to show the liquidity of a ticker in a diagram about the stock exchange.
Usually a legend is a passive object, located in a zone of the graphic that is void or meaningless. But when the legend becomes interactive many things can be improved regarding the understanding of the information behind the graphic.
An example of this is Gapminder_World a tool that enables the visualisation of many indicators of a great deal of countries. Among them we can find, for example, the yearly income per capita, the number of physicians per 1000 people and many more. The data covers the period from 1975 to 2004. The graphics these tools shows are indeed quite simple. A bubble scatter plot in the end. Its power comes from the fact that the legend surrounding it is completely interactive.
The title of the ordinate and abscissa axes allow the user to select any of the available indicators. A slider bar makes the selection of the data of a particular year possible but you can also create an animation of the data through the whole time range with the "play" button. Every circle represents a country. Its colour corresponds to a specific continent or geographic area. Hovering the mouse over the mapamundi (top right of the chart) the countries of the same zone begin to blink.
The size of each circle can be controlled with the pat of the legend that shows its meaning (down to the left of the chart), just by moving two little bugs that lie below the scale that indicates the amount corresponding to the smallest and maximum sized circles. Finally each country can be identified by checking its check box in the scrollable list to the right of the graphic. This action highlights the corresponding circle and makes all the others transparent.
This way what could just be a simple collection of charts ordered by date becomes a very powerful analysis tool. Only by seeing the evolution of Rwanda through time gives a very painful feeling of how the genocide dramatically diminished the life expectancy, which has taken a long time to recover. Other many powerful analyses are feasible too, that we leave to the interested reader.
It's remarkable the power that an interactive legend offers us taking into account what a simple thing it appears to be. In fact what can amplify the cognition and hence the possibilities of enhancing the understanding and analysis of the data underlying the graphic is not limited to the legend itself but to the interactivity of all the elements of it, from the titles to the scales through the colours and the keys for interpretation of the symbols.
Are there some rules or elements that could allow us to create intelligent legends in an optimal way?
According to the article Smart legend - Smart Atlas! by Sieber, Schmid and Wesmann from the Swiss Cartographic Institute at the ETH in Zürich, the legend can be incorporated in the map or appear in the form of an independent panel represented in a separate area. This last form has a greater potential for visualisation and analysis. From the analysis of the authors about the state of the art of interactive legend design we can consider seven interrelated components.
All these components can be interactive simultaneously since they complement one another. Depending of the properties of the graphic in question some of them can be lacking or be passive.
In any case the existence of those components don't guarantee by themselves alone a usable and agile interactive legend. To get this we need to use methods that confer it power. Some of the suggested ones in the same article are:
The appearance and experimentation with new interactive legends will for sure give rise to new refinements and methods that will empower even more our graphics legends. A component that we probably don't usually give the attention it deserves, and that can convert a bland graphic into a powerful tool of exploratory analysis.
Links of this issue:
Subscribe to the free newsletter