Consider this slope graph viz.
Tricky eh. One of the problems with Slope graphs is that they get messy very quickly. This is turn makes it hard to decipher whats going on. You only have a small window of engagement with your audience so making them work hard for the answer just isnt on. So here's a quick rule of thumb.
If the number of lines in the chart is greater than 5 then you need to have interactivity/highlighting/colourcoding
If its less than 5 then you can get away with a static display.
Remember, you are creating a dataviz, not a game of pick up sticks.