The purpose of the "ideal" trend in burndown charts is to show the progress of the team if they were working in a perfectly sustainable and constant and focused delivery pace. That hardly ever happens of course. Sometimes you're stuck and need help, sometimes you go faster than expected etc... And that's the whole point. This "ideal" line in burndown charts lets you detect very quickly if you are susceptible of running late, or if you're running early. The earlier you know that the better.
I sense there is an underlying problem behind your question. I don't believe burndown charts are your problem, they're just a tool. When you say "that does not represent what the team can do", do you mean your team usually runs late? How do you deal with velocity in your team and company? How about lowering velocity expectations the next sprint?
As for "off-topic" tasks, I believe your scrummaster or your agile coach should be there to protect you from it. Her job is to protect you from being driven away from the next most important thing.
So I guess the question is not wether there is a tool that matches how things are working in your team but rather, "can you tell us more about how your team works?", then we might be able to help you find some answers.
Back when I started digging in all this Agile stuff, I got a lot of good haha moments reading the book "Agile Estimating and Planning" by Mike Cohn. You might want to take a look at it.
Hope that helps.