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### One-line summary [问题简述]
chart data with null for step charts. if we connect nulls using connectNulls
property,
it is showing as line chart not as line chart
### Version & Environment [版本及环境]
+ ECharts version [ECharts 版本]:
+ Browser version [浏览器类型和版本]:
+ OS Version [操作系统类型和版本]:
It should show as step chart
### Expected behaviour [期望结果]
### ECharts option [ECharts配置项]
<!-- Copy and paste your 'echarts option' here. -->
<!-- [下方贴你的option，注意不要删掉下方 ```javascript 和 尾部的 ``` 字样。最好是我们能够直接运行的
option。如何得到能运行的 option 参见上方的 guidelines for contributing] -->
```javascript
option = {
title: {
text: 'Step Line'
},
tooltip: {
trigger: 'axis'
},
legend: {
data:['Step Start', 'Step Middle', 'Step End']
},
grid: {
left: '3%',
right: '4%',
bottom: '3%',
containLabel: true
},
toolbox: {
feature: {
saveAsImage: {}
}
},
xAxis: {
type: 'category',
data: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
},
yAxis: {
type: 'value'
},
series: [
{
name:'Step Start',
type:'line',
step: 'start',
connectNulls:true,
data:[120, null, 101, 134, 90, 230, 210]
},
{
name:'Step Middle',
type:'line',
step: 'middle',
connectNulls:true,
data:[220, 282, 201, 234, null, 430, 410]
},
{
name:'Step End',
type:'line',
step: 'end',
connectNulls:true,
data:[450, 432, 401, null, 590, 530, 510]
}
]
};
```
### Other comments [其他信息]
<!-- For example: Screenshot or Online demo -->
<!-- [例如，截图或线上实例 (JSFiddle/JSBin/Codepen)] -->
[ Full content available at:
https://github.com/apache/incubator-echarts/issues/8921 ]
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