fix(axis): Fix bar draw for inverted y/y2 axes

[netil]

Update getting y/y2 axes min value.
Side effect caused by #1323

Ref #2503

Issue

Details

bb.generate({
  data: {
    columns: [
	["data1", 30, 200, -100],
	["data2", 50, -50, 70]
    ],
    type: "bar",
    axes: {
	data1: "y",
	data2: "y2",
    }
  },
  axis: {
    y: {
	inverted: true
    },
    y2: {
	show: true,
	inverted: true
    }
  }
});

From the above option, will be drawn as:

[coveralls]

Coverage increased (+0.01%) to 90.257% when pulling a507f1f on netil:inverted#2503 into d3cc268 on naver:master.