Add elements dinamically via scripting

Migrateduser

<p>Hi all.<br><br>
I am trying to acommplish something simmilar to the screenshot (the grey and blue rectangles).<br><br>
Also I have a tradeoff: there is no cap maximum for the rectangles ammount, they must be generated dinamically.<br><br><br>
How can I do so?<br><br>
Thanks in advance.</p>