If "most work per day" is what he wants, then I'd bet that a bar chart - instead of a line chart - is likely to be the best bet. You can compare each of the four employees on a day-to-day basis that way...but you're still going to have to decide which way to do that, be it total files opened, percent of total that day, or whatever.

However, I suspect strongly that this kind of advice is outside the scope of this kind of forum, which is really more about "How do I make Excel work the way I want it?", rather than "How can I guess what my boss wants me to do?"