My original answer to this question has recently fallen into the third circle of hell because of a contrast I made between physics and biology which seems to have offended some biologists. I shall let that rest, but provide a new answer to make one point that I feel some biology students need to absorb.
The use of the word ‘rule’ should be regarded with great caution in biology. There may be some rules or laws in biology that justify the name (e.g. those that apply to classical genetics), but in many cases, the term is misused.
I maintain that the word ‘rule’ was misapplied by Chargaff to what was actually an observation. In science what is important about observations is how their interpretation can lead to an understanding of fundamental processes — in this case, the base-pairing in the double-helical model for the structure of DNA in many chromosomes.
This is what the modern student should focus on — the understanding of the fundamentals — so that when faced with observations about a particular chromosome that are inconsistent with the double-helical model he considers whether the fundamental model of the chromosome might be different in this case, rather than frets about some incomprehensible ‘rule’.
In the case of 𝜙𝑋174 this leads to the answer provided by @MarchHo, that the chromosome is single-stranded. Another possibility one might have considered would be that there were other bases than ATG and C that had not been detected (we now are aware of chemically modified bases in genomes). The fact that this would have been incorrect in this case is irrelevant — it would be the way one would want a student to approach observations.
Footnote: History and Science Education
I am not against making students aware of the experimental basis of our current views of molecular biology, and (given my age) have been in a position to do that for a number of posts. In general, it is a difficult task because so much is known. However, I feel that the teaching of ‘Chargaff’s rules’ (and Crick’s wobble ‘rules’) is often done without due reflection because they have a certain scientific celebrity. As a teacher one needs to have the courage of one’s own convictions. I feel it helps students if one always refers to ‘Chargaff’s observations (his so-called rule)’ and ‘Crick’s wobble predictions (his so-called rules)’.
And it seems that I’m not the only one. In the 5th edition of Biochemistry by Berg et al. we have no mention of Chargaff’s ‘rules’. Instead we have:
“In 1950, Erwin Chargaff reported that the ratios of adenine to
thymine and of guanine to cytosine were nearly the same in all species
studied. Note in Table 5.1 that all the adenine:thymine and
guanine:cytosine ratios are close to 1, whereas the adenine-to-guanine
ratio varies considerably. The meaning of these equivalences was not
evident until the Watson-Crick model was proposed, when it became
clear that they represent an essential facet of DNA structure.”