So after a couple of weeks off blogging due to Christmas, I’m back providing solutions for #WOW2021! New contributor Candra Mcrae started off the year with this gentle workout focussing on table calculations. For those who have followed my previous blogged solutions, you’ll probably recall that table calcs don’t phase me as much as they do some – I started using Tableau before LODs existed, so they were the only tool at my disposal in the past. That said, I’m still not such an expert that I get it right first time – there’s often plenty of trial and error as I choose which fields I want to compute by, although I’ve got much better at this since I read Andy Kriebel’s Table Calculations Overview a couple of years ago.

As with any challenge that involves table calcs, I tend to start by building out a tabular view of all the data I’m going to need to build the viz. This ensures I can validate the data much easier and set the table calculation settings to what I need.

So let’s start….

For the line chart itself, we’re simply going to be plotting Year against Food insecurity which has been formatted to 1 decimal place and displays the % as a suffix

We will also need to capture 2 other values which will represent the coloured circles. Parameters are needed to help identify what these circles will be.

pSelected Year is an integer parameter that can be built by right clicking on Year and selecting Create -> Parameter. This will populate the parameter with all the values of the Year field. Default the value to 2018 and adjust the formatting of the displayed value so that it is 0 decimal places and include thousand separators is unchecked.

With this parameter, we can capture the value associated which is represented by the pink dot in the viz

Selected Year %

IF [Year] = [pSelected Year] THEN [Food insecurity (includes low and very low food security) Percent of households] END

It’s a bit more work to identify the black dot, as this will vary based on another parameter

pComparison

I created this an integer parameter storing the values 0, 1 & 2 (defaulted to 2), but aliased for display as First Year, Most Recent Year, Previous Year. I’ll be using this is an case statement/if clause later and comparing integers to strings is much more efficient in Tableau.

I like to take things step by step, especially when there’s table calcs involved, to ensure all the values I’m referencing are correct, so rather than identifying the selected value in a single calculated field, I’m using multiple.

Firstly I want to identify the 1st year in the dataset (without hardcoding).

First Year

WINDOW_MIN(MIN([Year]))

This will store the value of the earliest year (1995) against every row in the data that is outputted in the view (a bit like you would get with {FIXED : MIN([Year])}, but this is a no LOD challenge).

From this I can work out

First Year %

IF MIN([Year]) = [First Year] THEN MIN([Food insecurity (includes low and very low food security) Percent of households]) END

Notice the MIN() functions used in this statement, as opposed to the Selected Year % above. This is because First Year is a table calc which is an aggregation, and subsequently other fields referenced in the calculation also need to be aggregated. In this case other aggregations such as AVG, MAX, ATTR would also suffice.

Similarly, I’m going to derive the Latest Year % with

Latest Year

WINDOW_MAX(MAX([Year]))

Latest Year %

IF MIN([Year]) = [Latest Year] THEN MIN([Food insecurity (includes low and very low food security) Percent of households]) END

Finally, I’m also going to work out the value for the previous year

Previous Year

[pSelected Year]-1

Previous Year %

IF [Year] = [Previous Year] THEN [Food insecurity (includes low and very low food security) Percent of households] END

With these 3 % fields, I can now create a fourth field which stores the value of the % I want to compare with

Selected Comparison %

CASE [pComparison]
WHEN 0 THEN [First Year %]
WHEN 1 THEN [Latest Year %]
WHEN 2 THEN SUM([Previous Year %])
END

This ultimately just stores the 2nd value required for the black dot. This could all have been written within this single calculation, but I find it easier to troubleshoot if things are broken down a bit.

Putting these onto a table we can see how the values in each row change, as the parameters are changed.

Note, for all the table calculation fields (all denoted by the triangle symbol on the pill), I have explicitly set them to Compute Using the specific dimension of Year rather than the default of table down. While this will give the same result, I prefer to be explicit to ensure the values don’t change if pills get subsequently moved around the canvas (in the case of the Selected Comparison % field, all Nested Calculations within the Edit Table Calculation dialog box need to be set).

This is enough information to build the main viz itself by

• adding Year to Columns (green continuous pill)
• adding Food insecurity to Rows
• adding Selected Year % to Rows next to Food insecurity
• then drag Selected Comparison % to the Selected Year % axis which will automatically change the display to have Measure Values on Rows instead. Set the table calculation setting to compute by Year

This chart can then be set to be dual-axis and the axes synchronised. The Food insecurities should remain as as line mark, and the Measure Values should be a circle. The colours, formatting and tooltip then need to be applied.

Now we need to go back to our table of data to build out other calculations. The requirement is a single viz, so we need to provide the % value of the selected year within the title of the chart, along with the difference from the comparison value. For this to work, we need to store the relevant values against every row in the data set.

We already have the Selected Year % value identified, but this is only captured against the row of the selected year in the data output. To get it to display against every row we need

Window Max – Selected Year %

WINDOW_MAX(MIN([Selected Year %]))

This is formatted to 0 dp with the % sign as suffix, as this is the field that will be displayed in the title. Added to our data table, with the table calc set to compute by Year, you can see the value replicated across every row

Similarly, we already have the Selected Comparison % captured, but in order to work out the difference, we also need to get this value against every row too

Window Max – Selected Comparison %

CASE [pComparison]
WHEN 0 THEN WINDOW_MAX([First Year %])
WHEN 1 THEN WINDOW_MAX([Latest Year %])
WHEN 2 THEN WINDOW_MAX(MAX([Previous Year %]))
END

Adding this in to the table (and remembering to set the table calc settings), you can also see the relevant value perpetuated against every row. Change the pComparison value and you’ll see the values change accordingly, but still the same on every row.

So for the difference…. at this point I chose to deviate from the solution published. We’re already dealing with a measure that is quantified as a % (as opposed to a measure that is quantified as £ say). So to get the % difference between two percentage measures, I simply chose to show the difference between the two values (ie selected – comparison). Some would refer to this a the point difference between the values. This makes most sense to me for this particular scenario. The alternative is to calculate the % difference in the more traditional way of (selected – comparison) / comparison as you may do if you were presenting the % difference between the value of sales in different years. But I personally found the result could be confusing to the reader .

Difference

[Win Max – Selected Year %]-[Win Max – Selected Comparison %]

BUT, the value displayed within the title is the absolute value (ie no -ve sign) and actually doesn’t display if the value is 0 (which you get if you select the year to be 2019 and comparison to most recent year). So I resolved this with

DISPLAY : Difference

IF [Difference] <> 0 THEN ABS([Difference]) END

which is formatted to 1 dp with % suffix

Finally, we have a symbol that is used to indicate if the change is +ve, -ve or remains the same.

Difference Indicator

IF [Difference] > 0 THEN ‘▲’
ELSEIF [Difference] < 0 THEN ‘▼’
ELSE ‘N/C’
END

Add this into the table too, once again remembering to set the table calc properties for all nested calculations.

So now you have these fields, you can add Window Max – Selected Year % and DISPLAY : Difference and Difference Indicator to the Detail shelf of the All marks card on the chart viz, and these will then be available in title dialog to add. Once again, ensure you set the table calcs to compute by Year.

You’ll also need to add some spacing in the title, to allow the parameter controls to be ‘floated’ into place on the dashboard. Getting the position right is really tricky. I positioned it very carefully on Desktop, but when I published, the controls were in completely different places. The easiest way I found to resolve this, was to use the Edit feature in Tableau public to edit the dashboard online and move the objects that way.

My published viz is here.

Happy vizzin’! Stay Safe!

Donna