score:6

Accepted answer

It looks like it's controlled by this CSS rule (nv.d3.css line 225):

.nvd3 .nv-axis.nv-x path.domain {
  stroke-opacity: 0;
}

Overriding that with your own CSS should show the line (at least, changing the value to 1 in the developer tools worked for me).

score:2

.nvd3 .nv-axis.nv-x path.domain, .nvd3 .nv-axis.nv-y path.domain {
  stroke: black;
  shape-rendering: crispEdges;
}

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