code: function (value, lineWidth, old, force, translatedValue) {
              var axis = this,
                chart = axis.chart,
                axisLeft = axis.left,
                axisTop = axis.top,
                x1,
                y1,
                x2,
                y2,
                cHeight = (old && chart.oldChartHeight) || chart.chartHeight,
                cWidth = (old && chart.oldChartWidth) || chart.chartWidth,
                skip,
                transB = axis.transB;
  
              translatedValue = pick(translatedValue, axis.translate(value, null, null, old));
              x1 = x2 = mathRound(translatedValue + transB);
              y1 = y2 = mathRound(cHeight - translatedValue - transB);
  
              if (isNaN(translatedValue)) { // no min or max
                  skip = true;
  
              } else if (axis.horiz) {
                  y1 = axisTop;
                  y2 = cHeight - axis.bottom;
                  if (x1 < axisLeft || x1 > axisLeft + axis.width) {
                      skip = true;
                  }
              } else {
                  x1 = axisLeft;
                  x2 = cWidth - axis.right;
  
                  if (y1 < axisTop || y1 > axisTop + axis.height) {
                      skip = true;
                  }
              }
              return skip && !force ?
                null :
                chart.renderer.crispLine([M, x1, y1, L, x2, y2], lineWidth || 1);
          }
docs: Create the path for a plot line that goes from the given value on
  this axis, across the plot to the opposite side
  @param {Number} value
  @param {Number} lineWidth Used for calculation crisp line
  @param {Number] old Use old coordinates (for resizing and rescaling)