Gridlines

Gridlines live under each axis:

axes: {
  left: {
    gridlines: {
      visible: true,
      color: '#aaaaaa',
      width: 0.5,
      dashArray: '5,5',
    },
  },
}

New charts default to a thin dashed style (0.5px, 5,5). Stroke style options include solid / dashed / dotted / dash-dot at multiple widths (including 0.5px).