What is the minimum voltage required to operate an AVR MCU at 8Mhz clock speed?

I want to run an AVR off the internal 8Mhz RC oscillator without any prescaller and want to make sure I have enough voltage for proper operation.

Answer

2.4 volts.

Calculated from this graph (which could have more thoughtfully placed guide lines!)...

Since the voltage is linear with clock speed between 1.8V and 2.7V (as per datasheet), it is a simple linear interpolation to get the answer...