Processing printing sensor readings and raising NaN exception at the same time
//convert to an int and map to the screen height
sensorValue = float(inString);

You can't convert a String to a Float just like that by casting.  You have to interpret the characters within the String.  Oh, and a float isn't an integer.

All you're doing there is trying to cast the address of the String object to a float, which it really doesn't like, hence you get NaN.

Try:

int sensorValue;
sensorValue = inString.toInt();