Hello Mazher,
If you want to see a rectangle that represents the task bar, you can use the addTaskLayer method that allows displaying any HTML elements in the timeline:
https://docs.dhtmlx.com/gantt/api__gantt_addtasklayer.html
Here is an example of how it can be implemented:
https://snippet.dhtmlx.com/frltk1hk
If you need something different, please describe how you expect it to look and work. You can share a picture that shows how you imagine it can be.
Gantt doesn’t calculate the ratehr values. If you added it as a task property, Gantt treats that as a custom parameter and doesn’t do anything with it. If you want to change the value, you need to implement a custom solution by using the Gantt API and Javascript.