If your chart looks similar to this, with a lot of data for one channel but only a little data for the others, then there are a few possible answers.
It could be that there is no data for channels 2 and 3 before 08:30 on April 19th. However, before you panic, it is probably best to check the settings for this chart widget:
In this case we can see that channel 1 and channel 2 have different spans set. This will mean that channel 1 will use the dashboard span which may not match the span for the other channels in the widget. Make sure that all channels' spans in the widget match and the problem should disappear.