When you combine plotm (or other *m.m functions in Mapping Toolbox) and geoshow, you need to define axis in GEOSHOW function. Also, set DisplayType option as 'point' in GEOSHOW (default value is 'line').
ax = worldmap('angola');
load coastlines
setm(ax, 'Origin', [0 20 0])
plotm(coastlat,coastlon);
lat = -8;
lon = 13;
geoshow(ax, lat, lon, 'DisplayType', 'point', 'Marker', 'o')
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/177197/image.png)