A plot diagram visually represents the exposition of the story; the rising action; the complications; the climax; the falling action; and the resolution of the story. In "Miss Brill," the exposition--the introduction of the character(s), plot, setting, action--occurs in medias res, which is in the middle of on-going action, and introduces us to the setting, the conflict, the action and to Miss Brill and the fox fur necklet that is at one and the same time part of the setting, a character to be reckoned with, and a part of the plot conflict.
It is a subtle part of the setting because it is really part of where the action and conflict occur ("It's her fu-ur which is so funny,"); character because to Miss Brill, it has a personality and importance of its own ("Little rogue! Yes, she really felt like that about it. Little rogue biting its tail ..."); plot conflict because it is the vehicle through which the truth of her presence is revealed to her (""It's exactly like a fried whiting." / "Ah, be off with you!" said the boy [to Miss Brill] in an angry whisper.")
The rising action, which would be notated on the upward left-hand slope of your pyramid diagram, consists of Miss Brill's walk to and observations at the Jardins Publiques, including the little girl, the conductor's new coat, and the conversations she listens to, like that about the woman's possible new spectacles. A complication would be the play that she perceives them all to be actors in.
The climax occurs when the "boy and girl ..., [who] were beautifully dressed; they were in love [and the] hero and heroine" of her little jardins publiques theatrical, break her heart and shatter her illusions by their cruel assessment of her, reaction to her, and remarks directed at her. From this point on, the rest of Miss Brill's day (perhaps life) is fixed and determined.
The falling action occurs as she leaves--we are not told of her departure--and walks home past the bakery. The narrator's report of her Sunday habits regarding the bakery, honey-cakes, raisins, and tea kettles constitutes the occurrences of the falling action. The resolution occurs when her hope is dashed and her dream crushed, when Miss Brill encounters her red eiderdown, the fox's little box, her closet-like room, and her bed.
[Miss Brill] went into the little dark room--her room like a cupboard--and sat down on the red eiderdown. She sat there for a long time. The box that the fur came out of was on the bed. She unclasped the necklet quickly; quickly, without looking, laid it inside. But when she put the lid on she thought she heard something crying.