I tried the following:
⇒but I get $⇒$
\impliesbut I get $\implies$
\Rightlongarrowbut I get $\Rightlongarrow$
\Rightarrow, I think--I use this online equation editor when I don't know what TeX to use.
Also, just because
\xxx shows up as
[\xxx?] doesn't mean it's wrong, just that Google Charts doesn't know what to do with it.