Describe the purpose

What does the image convey here? A product photo needs the product; a decorative divider needs nothing at all and should carry an empty alt attribute.

Keep it short and specific

A sentence is usually enough. "Chart showing uptime rising from 97% to 99.9% after migration" beats both "chart" and a paragraph.

Do not stuff keywords

It reads badly aloud and search engines discount it. Alt text written for a person is the version that performs better anyway.