You see how in this gauge the 70% progress is shown in green vs the 100% total that is in grey? It looks as if the green is "covering" the grey, yes?
How can I do this in a bar chart like this image below, where orange is the total and blue is the percent completed so far?
You're looking to create a cumulative series in your chart, it sounds like? It might depend on the data. Where is the blue/orange chart coming from? Is there a "% completed" field in your data already?
There's no built-in way to do this that I'm aware of, but you could accomplish it with a Data Expression. You'd need to iterate over each feature and calculate the % of the total based on all preceding values. Depending on the number of values in the table, it could take a while to calculate, but it wouldn't need to be terribly complicated.