Category label can get cut off for rotated graph with only 2 categories

* **C3 version**: 0.4.21
* **D3 version**: 3.5.8
* **Browser**: Chrome 64.0.3282.167
* **OS**: MacOS 10.13.3

I have a rotated graph so that the category labels are displayed to the left of the chart. Multiline is set to false so that the labels are on a single line. If there is 3 or more categories, the space for labels are calculated correctly, however if there are only 2 categories only the first label's width is considered. If the second label is longer than the first label it will extend outside the svg area and become cutoff.

Demo on codepen: https://codepen.io/MacArmstrong/pen/paKBMg

`var chart = c3.generate({
data: {
columns: [
['Values', 10, 20]
]
},
axis: {
rotated: true,
x: {
tick: {
multiline: false
},
type: 'category',
categories: ['Normal Label', 'Longer Label Name']
}
}
});`