This produces an error, but it works well at http://geojson.io (this is a line near Gabon)
<mapframe width=600 height=600> { "type": "Feature", "id": "Q1000", "properties": {}, "geometry": { "type": "GeometryCollection", "geometries": [ { "type": "LineString", "coordinates": [ [ -20.78146426141457, -20.24797277602244 ], [ 20.80974825712424, 30.094641604394508 ] ] } ] } } </mapframe>