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Mapbox choropleth map with customized area

Hi,

I try to make a choropleth map for the orange part in the following plot.


This scattermapbox is salt concentration (data from NASA resolution is 0.625 * 0.5) and I set the condition that salt concentration should be higher than a certain level so finally I have this plot.
I found some examples which deal with countries or counties. For my problem here, I only have the geographic coordinates and the region is kind of random.
As I understand, the idea of choropleth is to color a region with its boundaries’ coordinates but I don’t know how to generate the geo file.
Any comments will be appreciated!

Thank you!

Chang