x_axis = [Plot.ruleY([0], { x1: 1, x2: 8, dx: gap, strokeWidth: 0.5, stroke:'#999' }),
Plot.ruleX(ticks, { x: d => d, y: 1, insetBottom: -5, dy: 25, dx: gap, stroke:'#999' }),
Plot.text(ticks, { x: d => d, y: 1, dx: gap, dy: 25 }),
Plot.ruleX(ticks, { x: d => -d, y: 1, insetBottom: -5, dy: 25, dx: -gap, stroke:'#999' }),
Plot.text(ticks, { x: d => -d, y: 1, dy: 25,dx: -gap }),
]