After changing the Speed In/Out Override for an interface to 20,000,000 the interface stops displaying data
By setting the speed to 20,000,000 from a speed greater than 20,000,000 you are telling CAPM to use a different set of counters (64bit vs 32bit). In this case, the interface was not displaying proper data on the low speed metrics and thus dashboards began displaying "No Data to Display" errors.
High Speed Interface Vendor Certification formula:
(CalculatedSpeedIn > 20000000 && isdef(ifHCInOctets) && ifHCInOctets > 0) ? (ifHCInOctets*8) : (ifInOctets <= 786432000 ? (ifInOctets*8) : null)
Instead of overriding the speed to 20,000,000 override it to 20,000,001 as the cutoff in the vendor certification is anything > than 20,000,000 will use the 64bit OID values.