read keyPressed in the reference please.

have a global var

1. String enteredText = "";

When there is a key (test if is X or C)

1. enteredText = enteredText + key;

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
this stuff I posted is misleading!!!!!!!
test the length of
enteredText with

1. valueLength = enteredText.length();

and apply the value valueLength (multiply by 60 or so) to your bar

see
http://www.openprocessing.org/sketch/58822
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I'll check it out, thanks.

I was trying to do it for a while but I don't understand how you test if "X" is pressed after "C" and vice versa. Can you please explain me with the whole code? because it will be much easier to understand

Here's what I did so far, but if I continue pressing one key it increases the value:

1. String enteredText = "";
2. int valueLength;
3. void setup() {
4.   size(400, 400);
5. }
6. void draw() {
7.   println(valueLength);
8.   if (keyPressed){
9.   if (key=='x' || key=='c') {
10.     enteredText = enteredText + key;
11.   }
12.  
13.   }
14.   valueLength = enteredText.length();
15. }

OK, I'll try to help you out. 

keys in order to fill up an empty bar

sorry, I misunderstood your "empty bar" - my link to openprocessing is misleading ....
I marked it above with !!!!!! as misleding.

Sorry!   

ok but x alone increment (if after a C) or only c increments?

here, a version where both increments if in the right order...