Ciao,

I think it is not possible without modifying the library.

Anyway I did it quite easily by modifying the library (the biggest annoyance is to import the library and recompile it):
1. I downloaded the source code
2. I imported the project into Eclipse
3. I modified a couple of classes
4. I re-compiled the jar
5. I replaced the jar in C:\Users\filippo.panelli\Documents\Processing\libraries\gicentreUtils\library
6. I restarted Processing

What I changed:
in AbstractChart.java I replaced:

1. protected DecimalFormat[] axisFormatter;
   

with

1. protected Format[] axisFormatter; 

In the constructor I replaced:

1. axisFormatter    = new DecimalFormat[MAX_DIMENSIONS];       
   

with

1. axisFormatter    = new Format[MAX_DIMENSIONS];

then I added a new method by copying and pasting setFormat, I called it setDateFormat. It is the same code as setFormat with only 1 difference in bold below:

1. protected void setDateFormat(int dimension, String format)
2. {
3.     axisFormatter[dimension] = new SimpleDateFormat(format);
4.  ...
   

Last point, in XYChart. java I added to new methods similar to setXFormat and setYFormat, but called setXDateFormat and setYDateFormat, I show here the code for x-axis, you can figure out the one for y-axis:

1.     /** Sets the numerical format for numbers shown on the x-axis.
2.      *  @param format Format for numbers on the x-axis.
3.      */
4.     public void setXFormat(String format)
5.     {
6.         setFormat(0, format);
7.     }
8. 
   
9.     /** Sets the date format for dates/times shown on the x-axis.
10.      *  @param format Format for numbers on the x-axis.
11.      */
12.     public void setXDateFormat(String format)
13.     {
14.         setDateFormat(0, format);
15.     }

That's all.

Ciao
Filippo