Fascism is totalitarian.
Communism is totalitarian.
This has nothing to do with "aspects." It goes to the very heart of what both communism and fascism are. Totalitarianism is their shared ground of being. All else is secondary and incidental.
A cobra and a rattler may have different markings and adornments, but they are both poisonous snakes and they will both kill you. And they don't care if you can tell them apart by their surface features. To classify communism and fascism as "diametrically opposed" is as pointless as a biologist classifying rattlers and cobras as two unrelated species because one has a rattler and the other has a hood.
The whole purpose of classification is to group closely related entities together in the most meaningful and useful way.
The political spectrum to which you and most western academics subscribe, and which was conceived and developed by the Soviets in an effort to disassociate themselves from fascists, is not logically consistent or useful. It is a product of the "ideological needs" of the left and, far from facilitating "serious discussion of political science," it is more of a hindrance than a help.
Where does anarchism fall on your political spectrum?
Source, please.
We're not the first to observe that a a political spectrum is often inadequate. Here's an interesting model that seems to capture personal freedom and state control within a system that describes communism and fascism along the same lines as you:
