That thing is created with ui_template, showing pretty simple html div on top of the image.
The technique can be found here Dashboard tips. Let the ui_text act as level gauge ... DIY gauge examples
But as I understood, you are creating "SCADA - like" thing with the SVG. To advise anything solid, It takes to know a lot more of it. How it is built, what nodes you are using, what kind of communication you have created and so on. Maybe explore existing stuff and then find some solutions. Otherwise it will be just random recommendation with no actual value for you.
