Cannot stack bars on time x-axis

### One-line summary [问题简述]

Stacked bar charts collapsed since 4.0.3. See screenshot.

### Version & Environment [版本及环境]
+ ECharts version [ECharts 版本]: 4.0.4
+ Browser version [浏览器类型和版本]: Chrome 64
+ OS Version [操作系统类型和版本]: macOS High Sierra

### Expected behaviour [期望结果]

Bars able to stack on top of each other without overflowing issues.

### ECharts option [ECharts配置项]

```javascript
option = {
grid: {
right: '8%',
left: '8%'
},
xAxis: {
type: 'time',
minInterval: 1000 * 3600 * 24
},
yAxis: {
type: 'value',
min: 0,
max: 100,
name: '%'
},
series: [
{
type: 'bar',
stack: 'adherence',
data: [{
value: ['2018-02-21', 6, 2, 33, 67]
}],
encode: { x: 0, y: 3 },
barMaxWidth: 25,
barCategoryGap: '50%',
cursor: 'auto',
itemStyle: {
normal: {
color: 'red',
opacity: 0.5
}
}
},
{
type: 'bar',
stack: 'adherence',
data: [{
value: ['2018-02-21', 6, 2, 33, 67]
}],
encode: { x: 0, y: 4 },
barMaxWidth: 25,
barCategoryGap: '50%',
cursor: 'auto',
itemStyle: {
normal: {
color: 'green',
opacity: 0.5
}
}
}],
dataZoom: {
type: 'slider',
showDataShadow: false
}
}
```

### Other comments [其他信息]