This may also indicates a fashion shift. A parasols is an accessory, and it is often associated with women. But given that the look of an accessory can be altered (to be more manly), and Japan has a more fluid attitude towards gender fashion (especially the youth), leads me to believe that the adoption of a parasol can be both functional and esthetic.
My husband will not even wear a scarf - not even a masculine one that has gray and black - too girly for him - Pneumonia is much more manly. Therefore, an acceptance of an accessory has also to do with gender stereotypes.
Overall, this is an interesting observation to read about, and thank you for bringing out the charts - they are fun to ponder upon.