The dividing point between red and blue (indicated by the little sideways bowtie icon) represents the baseline value for whatever you're looking at. This is typically the average of the previous 5 time periods that match your current time filter (e.g. "last 5 Mondays").
If you see red, that means that the current value is greater than the average value. You can see details on the average and current value if you hover over the bar.
If you see no red, that means the current value is less than or equal to the average.
Thank you very much Scott , all cleared