Sounds like a scifi knock-off doesn’t it?
Junkcharts tells a different tale: The importance of explaining your chart: the case of the red 118
Great review of an temperature map for March, 2012.
Two take aways:
1) Even expert map makers (NOAA/NCDC) screw up and/or have a very difficult time communicating clearly.
2) Get someone outside your group to review maps/charts without any explanation from you. If they have to ask questions or you feel explanation is necessary, revise the map/chart and try again.