How to delete or reduce the gaps between data

I have some data which is only available for a few hours each day, when I plot those data, which use timestamp as x axis, it looks like:

```
| 1 2 3 4 10001 10002 10003 ...|
|<- log gap ->|
```
how to fix it looks like this:

```
| 1 2 3 4 10001 10002 10003 |
```