Continue Line chart based on a value until 100%

Hi, 

I have the line chart below and we have data until 23 July but I want the graph to continue until 100% based on the previous speed of the graph. 

It will be the prediction of when this will be finished. If you want to see what I mean in excel this is the chart in excel. 

The blue line is the extension line. 

We subtract %Completion of each day with the previous day and the average of all these days will be the average progress speed and I want to continue the line based on that number until it reaches 100%. 

Is there a way that this can be acheived?