I can find no reputable reference for Diplacus grandiflorus 'Maximus'.
Erythranthe guttata 'Maximus'

There is a series of hybrid Erythranthe (formally Mimulus) that is called Maximus but isn't listed on our database.
http://floranova.com/products/...

I suspect that the name got messed up when the Mimulus genus was broken up.

All of the images posted under this entry look consistent with the Erythranthe Maximus series.

@zuzu
@Calif_Sue

Yes, this is a case of faulty editing. M. grandiflorus became Erythranthe guttata. Mimulus aurantiacus var. grandiflorus is the one that became a synonym for Diplacus grandiflorus, so the change was recorded incorrectly in this entry, but it has been corrected now.