It sounds like using "Y-Value", Descending order of sorting should solve your needs. If it's not working as expected, I suggest posting a little sample of your app and someone can help you make it work as you need.
In QlikView, you could sort by Load Order, but unfortunately you can't do it in Sense... Preferably, I'd load "high", "average", and "low" as dual values with numbers behind each value. In that case they would sort nicely. Otherwise, you can construct a conditional expression with 2 nested IF statements. It's a bit ugly, but does the job. See attached.
If you'd like to play with colors, explore the Colors settings under Appearance - you can color by expression and do something similar there.