The data shown here comes from Flight Delay Analysis Dataset
This chart meant to show the "Percentage of flights delayed by more than or less than 15 mins". The color denotes whether the delay is greater than 15 mins or less than 15 mins. The tooltip shows the percentage delay w.r.t total delay of all flights, when you hover over it.
The questions that can be asked from this chart are:
The chart for Departures is shown here: Percentage of flights delayed w.r.t Departures.