An Arduino based "scanner" with ultrasonic detectors

Hi all!
I have constructed what I call a "DIY Scanning Tablet": a frame on whose perimeter I have laid a number of ultrasonic sensors. When an object is introduced within the area defined by the frame, the distance from the sensors is retrieved and the position of the object is calculated by trigonometric means. This information (the position of the object) is shown on a screen and can be used for other purposes (in my case to control a common anode led, whose color is changed depending on the position of the pointer).
I am using six HC-SR04; as I have mentioned in another post, their accuracy and precision are not good, and the results of the "scanner" are worse than what I was expecting.
Nevertheless, these are the results and failures are also worthy. I can identiy some ways to improve the project, but for the moment being I will not put more work on it.
Details can be found here:

Thanks . I am sure it will come in handy.