One is often required to make special maps that shows the distribution of certain features but one would prefer to use a custom symbol instead of the built-in circles, squares, triangles, etc. in the GMT plotting programs [plot] and [plot3]. Here we demonstrate one approach that allows for a fair bit of flexibility in designing ones own symbols.
Based on the hotspot locations in the remote file hotspots.txt (with a 3rd column giving the desired symbol sizes in inches) we lay down a world map and overlay red volcano symbols using our custom-built volcano symbol and [plot]. We do something similar with the bulls eye symbols. Without the fill option, however, they get the colors defined in bullseye.def.
using GMT
GMT.resetGMT() # hide
coast(region=:global360, xaxis=(annot=60,), yaxis=(annot=30,), title="Hotspot Islands and Hot Cities",
land=:darkgreen, water=:lightblue, area=5000, proj=:Win, figsize=15)
plot!("@hotspots.txt", symbol="kvolcano", markerline=:thinnest, fill=:red)
# Overlay a few bullseyes at NY, Cairo, and Perth
cities = [-74 40.45 1.0; 31.15 30.03 1.0; 115.49 -31.58 1.0; -56.16 -34.9 1.0]
plot!(cities, symbol="k@bullseye", show=true)