Article for programming data types

Solution 1:

I've been programming for over 40 years, and I think both styles are common and idiomatic. But as mentioned in several comments, you should be self-consistent within the sentence, so either

Transform Histogram to Graph

or

Transform a Histogram to a Graph

I believe the reason that it's acceptable without the article is because a type/class name can be used as a collective noun referring to objects of the given type in general. However, I think the form without articles is more often used in synopses, which tend to be terse, rather than the main text of the documentation; it's similar to the way newspaper headlines leave out noise words.

You could also write

Transforms Histograms to Graphs

but this tends to be avoided because the type name is a specific term in the programming language, and adding the plural suffix results in something that doesn't match that term. It could be distinguished using typography, e.g. Histograms, but this looks weird and can also be lost as the documentation is converted to different formats.