Help with refreshing contour plot dynamically

Hi,

I am trying to make a contour plot to monitor and display weather/environmental conditions (smoke, humidity, temperature, etc.) collecting from data sensors placed around a small field. The data is thus constantly changing and therefore the contour plots need to update whenever new data arrives. I am following the spectogram example and was able to show the first data. However, I cannot make the contour plot change when new data applied. Here is a simple example I use to test the contour changing part (and failed!)

>> inside the SpectrogramData class:

SpectrogramData(...) { value = 0.1; //set the initial data }

value(double x, y) {
return val;
}

double val;
...

>> now try to change it:
data.val = newvalue;
plot->replot();

This and various other ways like using invalidatePaintCache() or canvas()->update(), etc. still can get the contour plot to change.

Could you show me the way to do it?

Any help is greatly appreciated.