What Problem Does This Solve?
As the mouse hovers a feature (data point) on the map, highlight all occurrences of this type and dim the rest.
The map contains too many features so that it is difficult to identify them, distinguish them, pick out relationships, see patterns in the distribution.
The same location might be occupied by multiple features of different types and therefore visually obscure their existence.
When to Use This Pattern
Use to remove unwanted information that obscures the data the user is interested in. This will help to focus the user on data of interest through eliminating some of the visual clutter on the map and quieting most of it.
What’s the Solution?
The user hovers a data point. After a brief and deliberate delay the application should dim out the features that do not meet the desired criteria. This delay is necessary so that the user can move the mouse around as they wish without the map appearing to blink because the spotlight effect was triggered accidentally. A smooth dimming effect can add some additional delight to the experience.
You should provide a legend showing the classification types. This is important to illustrate to the user the relation between features or graphics on the map and their actual meaning and relationship. The data spotlight is then applied to both elements, the map and the legend, and can also be triggered either way.
Use a map tip to show additional information related to this data series or feature type. Use an info window that is triggered on click to interact with that series, e.g. add to selection or zoom to full extent.
Fast response and display times are critical, so optimally prepare and provide the data so it can be rendered and displayed on the client without the need for a round-trip to the server.
Why Use This Pattern?
Data Spotlight helps highlighting related features that are otherwise difficult to distinguish within the richness of the spatial data. It allows the user to dynamically remove visual clutter (data junk) and focus on the variable of interest without losing the context of the complete set if the application only allowed turning different types of features on and off.
Users may find this dynamic exploration behavior attractive which can lead to deeper engagement and further interaction with the application, especially when combined with (Reference) Map tips and Info windows.