You can add any diagram easily with ejs.
1. Click "loaded ejs as signed applet"
2. You will need to give permission for browser to load ejs into your computer (If it is the first time you click that button)
3. When EJS is fully loaded (It will take a few minutes for first time user).
4. You will find introduction/model/view panels. You can find out how the variables are defined, what kind of equations were used,...etc from model panel. The x,y coordinate for the big particle is defined as px,py in the model. The time is defined as t.
5. Click View panel to add a new window to display diagram.
1) add a dialog window (You can adjust the size and location).
2) add a plottingPanel to plot diagram.
3) add a trace element to display trace for (t,py)
4) assign variable t, py as x,y properties for trace.
5. Click run button to create a new simulation with the diagram you want.

I have created a [url=http://www.phy.ntnu.edu.tw/ntnujava/javaswf/sample2.swf]flash motion[/url] to demonstrate the above steps. However, the interface were shown  in Chinese Because I am running it with my Chinese OS.

You can also save the ejs locally.