[style-spec] update line-gradient doc, add an example expression #12882
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This PR updates the
doc
value for theline-gradient
property in the style spec.The previous information was ambiguous and only stated that this property defines a gradient.
The updated information explains that the value must be an expression that makes use of the
line-progress
input. Especially important is the note thatline-progress
is the percent distance along the line as measured in the webmercator projected coordinate space (not the length as measured on the wgs84 spheroid).(This bit of information was critical in debugging some misalignments when attempting to use
line-gradient
to visualize EV state of charge in the directions api playground)Also adds an
example
espression, the same expression used in the line-gradient GL JS example.