[R] add single points to a level plot

[lily li]

Hi all,

I'm trying to add single points with known coordinates to a level plot, but
could not find the proper answer. I got to know that layer() function is
good for this, but I don't know which package is related to this function.
The source is here:
https://stackoverflow.com/questions/28597149/add-xy-points-to-raster-map-generated-by-levelplot

but my question is a little different as I know the coordinates of the
single point, rather than a range. Thanks for any help you could provide.

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