Dynamic gauge legend for MapLibre

This simple control adds a dynamic legend component to a MapLibre GL map, where, for a given layer and property, the average value of this property for the features currently rendered is displayed and a tick points to its position in the gauge scale.

It's based in the gauge I implemented in the app Distanciamiento peatonal in Madrid (a map where the width of the sidewalks in Madrid are evaluated in terms of keeping the required social distancing), but made into a proper parametric control.

Check the live example here.

How to

It has no dependencies, but if you want to use it as an UMD bundle, you need to

yarn install
yarn build

You will find the deliverables in the /dist folder. Just reference the CSS in your html

 <link rel="stylesheet" type="text/css" href="dist/gaugelegend.css" />

If you want to use the global GaugeLegend object instead of importing the module, then you need to add the script too

 <script src="dist/gaugelegend.js"></script>

Then, in your script

const map = new maplibre.Map({ ... });

map.on('load', function () {
    map.addLayer({
        id: 'my_layer',
        ...
    });
    const gauge = new GaugeLegend({        
        layer: 'my_layer',
        property: 'my_property',
        colors: ["#007080", "#546e82", "#7f6984", "#a16287", "#c25689", "#e0448b", "#ff1d8e"],
        breaks: [7,17,32,57,129,253,334]
    });
    map.addControl(gauge, 'top-right');
})

And Ta-da!

Parameters

Param	Datatype	Description	Default
layer	string	The ID of the layer to be observed
property	string	The numeric property that will be observed
colors	string[ ]	Array of HEX colors to be used in the legend. Should match the ones used to style the layer
breaks	number[ ]	Array of breaks of the classification of the selected property. Must have the same size as colors
theme	enum['dark', 'light']	Avaliable themes	'dark'
size	string	Diameter of the control in CSS units	'100px'
unit	string	Unit symbol to be added to the displayed value	''
text1	string	Upper text	''
text2	string	Bottom text	''
trigger	enum['idle', 'moveend']	Map event that fires the control update. It defaults to idle as it's safer when the tiles are heavy or the number of rendered features is high. moveend gives a smoother experience but might give false results	'idle'

Custom theming

Custom themes can be used by adding the needed classes in your CSS:

.gauge-mycustomtheme {
    /* Using semitransparent background
    lightens the visual weight of the control */
    background: my_background_color;
}
.text-mycustomtheme {
    color: my_font_color;
    /* Adding a halo to improve readability 
    if semitransparent background was used */
    text-shadow: 0 0 3px my_background_color_no_transparency;
}

The tick won't be themed as its color doesn't rely on CSS, but this issue can be overrided:

gauge._tick.setAttribute('stroke', my_tick_color);

To Do

• Change CSS classes to maplibre namespace when available.

Notes

Example favicon by Nick Roach