On the 25th of November I gave a talk at the **South West England and Wales Tableau User Group** (#SWEWTUG) that covered two main topics:

- An introduction to the Tableau Beta Programs (e.g. Viz Animations, New Data Modelling Capabilities)
- Experimenting with new Tableau features

For the second part, I demonstrated some examples of Parameter Actions and Show/Hide Containers that were created during the 2019.2 beta phase, along with a variety of examples and inspiration from the Tableau community.

I then focused on the idea of **‘drawing on dashboards’**, which makes use Parameter Actions and the technique of using a parameter as a data source which I learned about from Jonathan Drummey‘s excellent blog post.

This post will focus on the techniques used for drawing on dashboards with sections on:

- Drawing lines and squares using the
**Line**mark type - Creating concentric circles of points on a map representing the distance from selected points using the
**DISTANCE()**calculation - Drawing connected lines between spatial points on a map using
**MAKEPOINT()**and**MAKELINE()**

### Using a parameter as a data source

For full details of how this works, I highly recommend reading Jonathan Drummey’s detailed blog post, which has examples and a workbook with many of the regex functions used in the technique, some of which I have edited and added to for the below use cases to work.

In summary, this technique uses text parameters and parameter actions to perform various operations, such as:

- Adding a value
- Removing a value
- Finding the position of a value in the parameter
- and many more…

This is done by storing a modified version of the current parameter value in a calculation that, for example, in the case of appending a value, concatenates the new value (of a selected point) at the end of the existing parameter value as can be seen in the calculation below:

In this example, [p.ln.Point List] is the text parameter that holds the series of x and y points that I will use to draw a line, and [Point ID] is the x and y values of the current point being hovered over (with some text on either side of each value to help with regex later) and finally a delimiter at the end.

### Drawing a Line

After reading Jonathan’s blog, I wondered if it would be possible to use this technique to draw lines on a dashboard, and it turns out it is:

This works by passing X and Y values of selected points in a 100 by 100 grid to a parameter using parameter actions and then drawing a line between those selected points:

The below diagram shows the parameter value after one point is selected:

Then after 4 points are selected:

A separate calculation [ln.Position in List] is then calculating the position of each point in the parameter using regex.

### Drawing rectangles

This technique can be adapted to draw rectangles by calculating the remaining two corners from the two selected points:

Below is the calculation used to draw the points for a square:

### Concentric circles showing distance from selected point

This was inspired by a tweet from Kent Marten some months back during the 2019.3 beta phase:

This gets a little trickier as, to use the **DISTANCE **calculation, spatial points need to be used, so the grid can’t just be a matrix of x and y numbers; instead, it needs to be a grid of latitude and longitude values, which can then be converted into spatial points using the **MAKEPOINT **function within Tableau.

To generate a grid of lat-long values I created a **Tableau Prep** flow, which takes as input the lat-long of the **top left** corner of the grid and the lat-long of the **lower right** corner of the grid and then calculates the specified number of lat-long pairs for all other points.

The spatial points are then added to the view and a single point can be selected using parameter actions – the values of which are passed to two parameters: [Source lat] and [Source long]. The DISTANCE calculation is then used to calculate the **distance between the selected point and all other spatial points in the grid**:

This is the final view:

Below is a dynamic view – selecting points in the grid and making the colour more transparent:

### Selecting multiple points and calculating the minimum distance

There were a few improvements I wanted to make to the above example:

- Make the map visible as it is currently obscured by the grid of points
- Allow multiple points to be selected
- For all other points in the grid, calculate the
**minimum**distance to the nearest selected point

To achieve this I added another grid layer to the dashboard underneath the map and **made the top grid layer transparent**:

I then used the parameter as a data source technique to be able to select multiple points sequentially.

Finally I created a new calculation to return the **minimum distance from each grid** point to any of the 5 selected grid points:

The below GIF shows how the grid of distance “circles” updating as each additional point is selected:

### Drawing connected lines between spatial points on a map

In this example I want to draw lines between cities on a map and calculate the cumulative distance as more cities are added to the ‘journey’.

The source data (Superstore Sales) has latitude and longitude data for the United States. I need to turn those into spatial points using MAKEPOINT so that I can then use the DISTANCE calculation to calculate the distance between all selected cities in the order they have been selected.

To do this, I need a **calculation to return the prior city that was selected** so that each line of my data will have the current city and the prior city. I can then use the distance calculation on each row for the current and prior cities.

The below calculation extracts the latitude of the previously selected city:

As the above is a string, I then need to **convert this to a float**:

This is repeated for longitude and I can then create the **spatial point for the previous point**:

Below is the final result:

### Summary

Jonathan mentions a number of possible use cases for using a parameter as a data source in his blog, such as “building games, creating surveys or quizzes inside a viz, dynamically re-sorting marks”.

The above examples, with being able to draw a route between points on a maps, in particular, I think open up some other interesting use cases.

### Workbook

The workbook with all the above example can be found on my Tableau Public profile here.

**Note**: at this time (Nov 2019), a Distance calculation containing parameters (or LODs) does not work with extracts, so **some** views will not render in Tableau Public. Download the workbook and switch to using live data source connections for full functionality.

I hope you’ve found this post interesting and if you have any feedback or thoughts on possible use cases, I’d be happy to hear your ideas. Thanks!

Marc