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Is there a simple way to display latitude/longitude point data like a 3D bar chart in KML, where the height is specified by a varying value?

I've seen some examples where the <extrude> element is used in conjunction with line-string polygon, but can it be done with a simple point/placemark?

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Extruding a point in KML will drop a line from the point to the ground if altimodeMode is absolute or relativeToGround.

The documentation on extrude on a Point states:

Specifies whether to connect the point to the ground with a line. To extrude a Point, the value for <altitudeMode> must be either relativeToGround, relativeToSeaFloor, or absolute. The point is extruded toward the center of the Earth's sphere.

See KML example with an extruded Point:
https://googlemaps.github.io/kml-samples/morekml/Point_Placemarks/Point_Placemarks.Extruded.kml
Extruded point in Google Earth

Alternatively, you may also want to consider scaling the icon image size and/or color correlating to the value or values you want to visualize.

If you want to create thematic maps then you can extrude a colored polygon at a given altitude to create 3-D bar charts with geo-spatial data. Details and techniques can be found here.
3-D thematic map in Google Earth

Here's blog on creating 3D bars in KML: http://blog.thematicmapping.org/2008/04/drawing-3d-bars-in-kml.html

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