Input text box to change scale limits

Im new to using processing to graph data from arduino. I am using the Aurdino in PID mode to control Heating element for my Brew stand. I am using the PID front found here http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1243714052.

I am in the midst of modifiying it to graph some more temp sensors. What I would like to do as add two text boxes that I can input min and max scale so I can change the scale on the fly as during the brew process I will target different temperatures.

The PID processing front end uses the controlP5 library. Is there an easy way to do this. I tried following

InScaleMinField=controlP5.addTextfield("Min Scale", 80,100,60,20);
  InScaleMin= InScaleMinField;  //InScaleMin is Called earlier in the code and is what sets the 
Min on the Y axis

But it gives me the error cannot convert from textfield to float. I can post the rest of the code if necessary but is quite long.

Thanks in advance.