The Sentence Diagram Generator will visually represent the grammatical structure of the sentence: "In the middle of the night, my dog barked at a mysterious figure, a man with a red mask."

1. Main Clause: "my dog barked at a mysterious figure"

   • Subject: "my dog"
   • Verb: "barked"
   • Prepositional Phrase: "at a mysterious figure"
     • Object of the Preposition: "figure"
     • Adjective: "mysterious"

2. Appositive Phrase: "a man with a red mask"

   • Noun: "man"
   • Prepositional Phrase: "with a red mask"
     • Object of the Preposition: "mask"
     • Adjective: "red"

3. Introductory Prepositional Phrase: "In the middle of the night"

   • Preposition: "In"
   • Object of the Preposition: "middle"
     • Prepositional Phrase: "of the night"
       • Object of the Preposition: "night"

This structure can be visually diagrammed using the Sentence Diagram Generator to illustrate the relationships between the components of the sentence.