I modified it with some of your suggestions.

I did not add slider for x,y because you can drag particle to any position with your mouse.
Or do you want numerical value for x and y?

The unit for B is T, for time is us, for m is $10^{-26}$kg, for q is $10^{-19}$C, for v is km/s,
it is cm for the coordinate unit.

You can read the coordinate when you press mouse inside the screen.
I can add ruler if you still need it.

Click the eye (image) to display the simulation

/htdocs/ntnujava/ejsuser/2/users/ntnu/fkh/changeGaussianField3_pkg/changeGaussianField3.propertiesFull screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
Download EJS jar file(1054kB):double click downloaded file to run it. , Download EJS source View EJS source