数轴

• d3.svg.axis - create a new axis generator.
• axis - creates or updates an axis for the given selection or transition.
• axis.scale - get or set the axis scale.
• axis.orient - get or set the axis orientation.
• axis.ticks - control how ticks are generated for the axis.
• axis.tickValues - specify tick values explicitly.
• axis.tickSize - specify the size of major, minor and end ticks.
• axis.innerTickSize - specify the size of inner ticks.
• axis.outerTickSize - specify the size of outer ticks.
• axis.tickPadding - specify padding between ticks and tick labels.
• axis.tickFormat - override the tick formatting for labels.