I'm not sure what "x" you're saying does not need a letter sign. However, your suggested transcription of "R sub y-axis" is correct.
As for the punctuation indicator. You do need it between "R sub y-axis" and a period. Please see Nemeth Code section 37.x, which deals with this issue. (The reasoning here could be that "R sub y-axis" is all one expression, which represents one value, so it is technical material and so needs to be separated from punctuation by a punctuation indicator.)