I was given a hand drawn whiteboard shot. Someone needs a scatter plot with radial references just like that. It made sense to me immediately. In a scatter plot, horizontal and vertical references seem no more enough. I would love to see such a chart in Tableau.
I know what is needed: polygon in dual axis with scatter plot. But I never did it before. It still took some research and optimizations to make it work. Bora Beran's blog is a great source of inspiration to me in my research.

1.Data preparation
To add radial references, we need to prepare your data first. That is, add a few rows to the data set. This can been done by Union in SQL.

Assuming we want 3 reference arcs, then we need 6 rows or 2 rows per arc. The 2 rows designate both start and end points for each polygon-based arc. Here we will use 102 points for each polygon. 102 points seem enough to make the arcs look smooth. More points can be added if necessary.

Note that there is a Reference column in the table which labels rows for arcs and data.
2.Drawing Arcs
This will use the data densification technique native to Tableau. The main steps are:
- create bins based on [Path] with step size = 1. This will create potentially102 bins.
- drag Path bin to Rows
- right click Path bin on Rows and select Show Missing Values.
- select Polygon as data mark
- drag Path bin to the Path shelf in Polygon data marks.

Then create coordinates for the arcs.
- X Arc and similar for Y Arc
- Drag X Arc to Columns and Y Arc to Rows. Both are table calculations. Make sure they are calculated along the Path bin dimension. We should see the arcs.

3.Creating the Scatter Plot
Use X and Y and the plot is easy. Dot Size can be any measure that will show in the size of each dot.

4.Dual axis both Arcs and Scatter Plot
We need to have a single horizontal axis variable for both charts. Here is X All:

  • IF ATTR( [Reference] )='Data' THEN ATTR( [X] )
  • ELSE [X Arc]
  • END
Note that we have to move X to the Details shelf in the scatter plot's marks card. This is necessary to display all the dots. Make sure X is a dimension by right clicking it. Also Y has to be a decimal type in order to sync with the other axis.

Voila, it's basically done.
Click the image to go to the interactive version.

5.Parametrization
Note that we use one parameter for each arc's radius. The radius can be changed to any value.

The number of points for an arc can also be a parameter. Here we just use a fixed number: 102.

There seems that the number of arcs can not be parametrized. We need to define the number of arcs a priori.

6.Generalization 
We assumed that the scales of both X and Y axis are the same. In some cases, this may not be true. Then an arc may not be a quarter of a circle. Instead, it is part of an ellipse. If that's the case, we have to calculate the coordinates differently.

7.Use cases
Forrester Research has been using scatter plots with radial references for years. Check them out: https://goo.gl/oJZ6p7

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Jake and I collaborated on a dashboard. He told me that he learnt a way to create an in-place help page in Tableau. He first saw it at a conference somewhere and couldn't recall who the speaker was. So I am blogging here about it but the credit goes to somebody else. If anyone knows who the original creator is, leave a comment below.

The key idea is to float a semi transparent worksheet on top of the dashboard, where a help text box is strategically placed on top of each chart. This way, we can explain how to view each chart and what data points are important, etc. This worksheet is collapsible by a show/hide button. 

Below I would like to show how this worksheet can be constructed.

1. Sheet with a single data mark.

  • Double click the empty space in Marks panel and add two single quotes. Make the null pill a text label. This creates a single null mark.
  • Set the view as "Entire View"

2. Create an show/hide button

  • Go to the target dashboard
  • Drag a floating vertical container to the dashboard, making it cover all the area of interest.
  • Drag the Single Null Mark sheet and drop it into the above container. Hide the sheet title.
  • Create an open/close button for the container and place the button at the top-right corner.

3. Add annotations

  • Format the sheet background opacity as 70% in the layout manager             
  • Select area annotations and place them anywhere of interest. 
  • Write help text and format it to highlight important messages.  
  • The text can serve as functional guide and/or insight guide.

Here is an example. Feel free to download the workbook and explore. Click the "i" button at the top-right corner to view the in-place help. 

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