If you want to place the label at the start of the line, try to use the following formula for x coordinate:

    x_at(0)

And use the following formula for y coordinate:

    y_at(0)

To align the labels at the center of the circle in your example, you need to use the `Alignment horizontal and vertical`:

[![enter image description here][1]][1]

In alignment `horizontal` write `'Center'`, and alignment `vertical` write `'Half'`. you will get the following result, which is exactly what you need:

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/km6bu.png
  [2]: https://i.sstatic.net/ZC1SE.png