Describe the bug

When grouping line charts via z, some lines are drawn where there should be none.
It seems to happen only in case of single element groups.

To Reproduce

Consider following example:

<script>
  import { Plot, Line } from 'svelteplot';
</script>

<Plot>
  <Line
    data={[{ x: 1, y: 1 }, { x: 1, y: 3 }, { x: 2, y: 5 }, { x: 3, y: 2 }]}
    x="x"
    y="y"
    z="x"
    marker={true}
  />
</Plot>

It connects dots from group 2 and 3.

See also https://svelte.dev/playground/f9a5cd905657450c9f6d7e935d5365f6?version=5.30.2

Expected behavior

Lines should only be drawn within a z group, not between them.

Here is corresponding example in Observable Plot

Plot.line(
  [{ x: 1, y: 1 }, { x: 1, y: 3 }, { x: 2, y: 5 }, { x: 3, y: 2 }],
  { x: "x", y: "y", z: "x", marker: true }
).plot()

and its output