I wanted to have a slim meter to mount under the dash with only the row of LED's visible.

The voltage range is 1.5 Volts for rich ( Lambda 0.70 , AFR 10 ) to 3.0 Volts for lean ( Lambda 1.44 , AFR 21 ).

For reference I added two additional LED's that are lit all the time.
One green for Lambda 0.86 (Maximum power).
One yellow for lambda 1.05 ( Best fuel economy ).

I used green LED's between max power and max fuel economy.
I used yellow LED's above the max power LED and up to lambda 0.75 (AFR 11). That is the range used for supercharged vehicles.
I used red LED's for values leaner than the Best fuel economy LED and for values richer than lambda 0.75 (AFR 11).