These alternatives may not offer all the features of OmniGraffle but can be a good starting point if you're looking for something free or more affordable.
Some downloads contain malware like the Sarcoma or Core variants, which encrypt your personal documents and demand payment to unlock them.
Instead, I can offer a long-form article on legitimate alternatives and ways to obtain OmniGraffle legally, such as:
One of the most common threats found in cracked Mac apps. It silently harvests your Keychain (passwords), browser cookies, and even cryptocurrency wallets within seconds.