X-axis tick labels truncated at end

* **C3 version**: 0.7.15
* **D3 version**: 5.15.0
* **Browser**: Safari 13.0.5 (15608.5.11)
* **OS**: macOS 10.15.3

See [this chart](https://latencyzero.github.io/COVID/) for an example. I specify the ticks to show along the X-axis as

```
axis: {
x: {
label: { text: "Day", position: "outer-middle" },
type: "timeseries",
tick: { format: "%b-%d", values: [ new Date(2020, 0, 22), new Date(2020, 1, 1), new Date(2020, 1, 15), new Date(2020, 2, 1), new Date(2020, 2, 15), new Date(2020, 3, 1), new Date(2020, 3, 15), new Date(2020, 4, 1), new Date(2020, 4, 15), new Date(2020, 5, 1), new Date(2020, 5, 15), lastDate ] }
},
}
```

It would be nice if C3 could adjust the position of the text label for the last tick on the right to show the full label, rather than truncating it off the right edge. Thanks!