I was amazed at this "scholarly" article, which to me seems like a long paranoid rant about how Plant based his vocal style and his manner of dress on Janis Joplin:
http://onlinelibrary.wiley.com/enhanced/doi/10.1111/jpms.12081/
What I take away from it is that Plant admired Janis and may have developed his hippie stage presence based on her personal style, but the fact is both of them based aspects of their vocal styles on the blues and Plant's voice is so different from Janis' that I find them hard to compare to each other. Plant's voice is often ethereal, and I find Janis Joplin's