I am receiving undefined when using charAt[i]

Two thoughts:

if (char != ".") { value += char; }
else { dpLoc = i; }

'data' always has a decimal point?

I2C1.writeTo(a, dpc);
  if(typeof dpLoc !== 'undefined') {
    I2C1.writeTo(a, dplo[dpLoc - 1]);
  }