I am attempting to create a shaded band that covers +/- 1 standard deviation on a view. (The SD in question is calculated on a larger population than the specific records in the current view, so the SD calculated by the "Distribution" option for reference line is not appropriate.) I have created, in the data source upstream from tableau, a field attached to each record with the SD of interest, and that SD*-1 as separate fields. They come into Tableau fine.
However, when I add one of them to a dual axis pane on the axis that has a horizontal bar, it fails because Tableau allows only one field, so it replaces the main field I have showing as a bar. When I add it to the other axis, it adds it as a shape, which I can live with because I can manipulate it to be not visible. But, I would like to make a reference line as a range from the SD to 0. I can not do that, because as shown in the third page of the attached PDF, the options for setting a value for the reference line include the field on the other axis (not useful in this case), and the combined field "Measure Values". I created a parameter called Zero, with a value of 0, to use as the other end of the range, but I cannot use a field as the outer boundary of my range.
A workaround is to create a parameter equal to the SD desired. I have implemented my concept using this on the second panel in the attached PDF. This works, but, my understanding is that parameters created from fields do not update when the underlying data is refreshed. So it is not an effective workaround in the longer term unless the parameter in the view is manually updated after each data refresh that underlies it.
If there is an alternate technique to do this that would refresh automatically, I'd be interested.
I think Tableau should:
1. Allow parameters to link to and refresh with fields.
2. Allow reference lines to be set to a field value even if the field is not used in a graphic on an axis.
3. Make separating the component fields in a "Measure Values" or Measure Names" field easy and forthright to accomplish.
4. Allow more than one field to be represented on the same row with horizontal bars. Using colors, sizes, and transparency, they can both be visible. In some cases, such as this one, a stacked bar is not appropriate or optimal.