In light of advice that Reed-Kellogg cannot handle this type of causative construction and the advice to make it up, I propose the following:
If you think about it, the sentence can be reinterpreted as “The hot weather made (it such that) she wanted to swim.” The only problem is what to do about “her”. It's an object of “made” but also a subject of “want”. So my advice is to show those relationships as simply and cleanly as possible:
- Set up the basic subject / verb / object structure for the main clause “weather made”
- In the direct object space, set up another S / V / O structure on stilts.
- Make “her” the subject of "want" as if it were a “she” as in the alternate interpretation.
- Put the infinitive “to swim” on stilts as the object of “want”.
- If you want, you could put “(her)” in the direct object area with arrows pointing to its reanalyzed position as the subject of “want”.
I've always thought the purpose of diagramming was to show you understand the structure of a sentence, the functions of its words, and the relationships between those words. Even if this is wrong, it shows you know better than to make an objective pronoun the subject of a verb not in the 3rd person, but also shows you see that “her” has both properties of an object and properties of a subject. (It's like asking what the part of speech of a present participle is. It is derived from a verb and has verbal properties, but it also functions as an adjective.)