Proportion of Fuel Type Consumption by Fuel Efficiency Rating

The chart shows the share of fuel consumed (HFO vs Diesel) 
within each fuel-efficiency band, 
where ratings are derived as quintiles of `distance / fuel_consumption` (km per kg of fuel). 
Higher km/kg → higher grade, with **A = best** and **E = worst**.
The standout finding is the **A grade**: 
voyages in the top efficiency quintile burn **100% Diesel and 0% HFO**. 
Every other grade (B through E) is a roughly even mix, 
with HFO floating between ~38% and ~55% and no clear monotonic trend across them.
The discontinuity between A and B is the dominant feature of the plot. 
It says that once a voyage clears the top-quintile efficiency threshold, 
the fuel choice is no longer ambiguous — it's Diesel.
This pattern is consistent with the physics: 
Diesel (a distillate fuel, equivalent to Marine Diesel Oil in shipping) 
has higher usable energy per kg 
than HFO, the residual fuel oil. 
Ships burning Diesel cover more distance per unit mass of fuel, 
so they cluster at the top of the km/kg distribution. 
HFO voyages, weighed down by lower energy density, populate the middle and lower bands.
**Caveat:** the chart shows correlation, not causation. 
Fuel choice is one of many factors that influence km/kg 
— route, ship type, weather, engine condition, and load all matter 
— so "A grade = Diesel" is best read as a statistical signature of the dataset, 
not a guarantee that switching fuels alone would lift a voyage's rating.