Display only one city on the map

Another newbie question.
I’m trying to make something like this viz for another city. I have centroids of clusters and I’m building boundaries of the clusters with a help of Voronoi diagram and then fill clusters with color.
I use go.layout.Mapbox for my layout.
At the moment everything is fine except some of my polygones get out of the city boundaries. Is there any way to display not the whole map but only one city as a layout, or to make a layer above filled with one color anywhere except selected city?

Thank you.