Will one 'C' grade due to health issues ruin my chances of admission into a top grad school?
You have shown that you have overcome great adversity and still achieved great results - this says a lot of positives about your character - of resilience and perseverance - two attributes that are critical for any graduate studies.  You had a cancer scare and still passed the subject despite the medical tests and the very justified anxiety.

Maybe, it is not an "ugly spot", but that C, and subsequent successes are a reminder of how much strength and tenacity you have shown.