There was a forum discussion about this several months back, but I've been unable to find it. What I'd like is for Tableau to be able to generate reference lines that are more than just horizontal or vertical bands based on the dimensions and instead are drawn as continuously changing values. This would allow calculated reference lines where the reference line y could be of the form y = mx + b, for example. A banded reference line could then be a continously shaded area, and would remove the need for the data reshaping necessary to "shade between the lines" as in the attached workbook in this forum post:. Another application of this feature would be that users could create color-banded "area charts" where the bands are a continuously changing % of the measure, whereas area charts right now (as of 7.0) require the color bands to be discrete dimensions.
Please note, the following is a duplicate idea and it's score should be included here: