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.

Gapminder World. Both charts depict the same but to the right we have used the interactive legend to change the size (and meaning)  of diameter of the minimun and maximum circle. Source: Screenshot by the author, as can be seen in the website Click on the graphic to enlarge it

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.

Gapminder World. Detail of the part of the interactive legend that allows the user to change the size preserving the value of the maximum and minimum circles.   Source: Screenshot by the author, as can be seen in the website Click on the graphic to enlarge it

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.

• Visualisation: defines how the data and/or elements are represented in the graphic or map. Here we can find a selection of symbols, sizes, colours, etc. in order to relate the different variables in an interactive way.
  
  
• Navigation: contains basically global information about orientation and navigation elements. For example coordinates, indication of the north, scale bar... Tools of navigation like zoom or pan are placed outside the legend.
  
  
• Analysis: this component enables exploratory analysis using, for example, histograms and diagrams statistics, data supresion (filtering) and data extraction (highlighting).
  
  
• Dimensionality: it is understood both in temporal and spatial sense. Spatial dimension is taken into account by means of the use of geometrical or 3D figurative symbols like cubes, spheres, etc. Temporal dimension is applied to indicate a certain point in time or the change that occurs in time (for example the time slider in Gapminder, an analogic clock, etc) 
  
  
• Themes: In thematic maps there could exist extensions to the legend to provide understanding of the theme, like special symbols, labels or textual descriptions. 
  
  
• General Information: contains elements that don't have a direct relation with the theme of the map or graphic like authorship, year of publication, copyright etc.
  
  
• Layout: characterizes the general organization of the elements of the legend. It incorporates the design of the panel and the graphic aspects of the same (fonts, separation lines, etc.) along with technical aspects like lineal or hierarchic structure (nesting of the different parts of the legend) 

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:

• An elementary method consisting of highlighting the element of the legend that refers to an attribute the user is focusing on. in the Gapminder example, making all the circles pertaining to the same region blink when the mouse hovers in the corresponding part of the mapamundi.
  
  
• Another method that provides potential is the possibility of changing the colour, size or any other visual variable in the legend and seeing a corresponding change applied directly in the graphic. For many users this constitutes a very intuitive way of acting on the visualisation at the same time that we get information about its data.  In the case of Gapminder we can change the size of the circles in the lower right legend and see the same change in the actual circles plus knowing the magnitude corresponding to each diameter. 
  
  
• The legend can allow us to select which information appears and which doesn't, or highlight those symbols of our interest, hiding or making the irrelevant ones transparent. In Gapminder this is done by choosing countries from the list.
  
  
• You can select select from a stack of information layers the one that we want to represent on the map or graphic. For example choosing the title of ordinates or abscissas in Gapminder we select which indicators will appear in the chart. In a thematic map we can select geology or population data for example 
  
  
• Another method enables the user to create his/her own legend by dragging symbols, colours or other elements to the same and associating them to the appropriate variables we want to represent. 

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.

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