I wouldn't think a diagonal line is a must for CRT. True, however, it is no longer used on LCDs. FCOM 3 differentiates as follows:
A)
CRT: blank, or distorted
LCD: blank with or without a large letter F displayed in amber, or distorted
B)
CRT: Diagonal line displayed
LCD: INVALID DISPLAY UNIT message, or
INVALID DATA message.
FD (the un-real)
taken from Display Unit Failure Abnormal Proc, non-ecam.