Chance is a feminine noun. However I have seen people write "bon chance" and not "bonne chance". Even Google Translate translates "good luck" to "bon chance". Is "bon chance" really correct?
"Bon chance" isn't valid in French for the reason you wrote in your question. Chance being a feminine noun, the proper spelling is "bonne chance".
Though, "Bon chance" is said as a "joke" (at least, in Quebec, don't know about the other french speaking places). It's a reference from the movie Taken. I don't know if this is universal, but this is where most people I hear say "Bon chance" got it from. (It happens at the end of the phone call between the kidnapper and Liam Neeson in the first movie)
There is only one correct way: bonne chance.
The n-gram you provide was queried against the English corpus, not the French one. There is no "bon chance" on the French one.
If you've seen it written "bon chance", it's possible that it's been misspelled by the writer, because bon can be pronounced like "bonne" in front of masculine word starting with a vowel, like in "bon appétit". But it's just a mistake.