Enhancements to world map with events

[unode]

This is to capture two unresolved points from #23

• Use a single color gradient instead of a viridis scale
• Apply a log transformation to the color scale

Originally posted by @mihai-sysbio in https://github.com/elixir-europe-training/ELIXIR-TrP-Training-Metrics-Database-Tango/issues/23#issuecomment-1651724511