You need to implement exactly the logic you're describing in code somewhere, whether it's using Vega, Transforms, or an external script that performs the summary. If your goal is to visualize this using Kibana's built-in area charts, then the data format you'll need is something like this:
{ timestamp: 1, status: 'completed', count: 4 }
{ timestamp: 1, status: 'ready', count: 2 }
{ timestamp: 2, status: 'completed', count: 1 }
{ timestamp: 2, status: 'ready', count: 6 }
Notice that in my example above, I've repeated all the status summaries multiple times for each timestamp interval. This might require some complex scripting to achieve.