Where is stored the touch screen calibration data?

I am using an eGalax 7" touchscreen, and it works, but axes are inverted, and need some calibration. I did it on raspbian image, and worked fine, so I think must be possible to do it on Volumio. But I have no idea if it is based on xinput, lslib, evdev… so the questions are:

-is there any calibration method for touch screens in Volumio?
-If not, where is stored the calibration data?

It would be great if we can use not only the official touch screen…