Day 19: Donut Charts

I want to begin today with a big thank you to the Tableau Public team.  I was honored to have the #Tableau30for30 series featured on their blog Friday.  It's been a fun journey so far, though I'm looking forward to May :) (for the weather, of course).  As for today, we're going to continue to show some out of the box chart types, though this one is kinda like playing with fire.  If I tell you how to do this one, you have to promise to only use it in the "right circumstance" (this is me channeling my inner Stephen Few).  Sometimes clients ask for gages.  We don't like to talk about it, but it happens.  We'll often show them a bullet chart and they'll go with that.  But sometimes that's not quite enough.  Up steps the donut chart.  Shaped like a gage, and showing the same amount of information as a gage, it can be quite the crowd pleaser.  Just promise you won't tell Stephen where you heard it.  Got it?  Good

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So here's the goal.  We have our actual and our target of (pick your flavor of measure/metric - today mine is) Margin.  A donut chart shows what percentage of your way to the target you are (or if you're over target, it shows that too).  Again, here's an example and where we're heading today:

To get there we're going to need to create three calculated fields, and leverage the dual axis solution from a little while ago.  If you're ready let's get going.

Today we're switching to the Sample Coffee Chain data (because it's day 19 and I need the caffeine).  The coffee chain data has a number of different metrics with actuals and budgets, the prefect use case for a donut chart.  Since we're going to have to create three calculated fields, let's start there. 

The first is going to give us the percent of Actual vs Target (logically named "Margin - Percent of Budget"):

The next thing we have to calculate is the remaining amount to get 100% of the budget.  Now, if we knew that the data would allow you to go over budget, then this calculation is as simple as: 
1-SUM( [Margin] )/SUM([Budget Margin] )

However, if there's anything I've learned from the government since of the time of Bill Clinton, it's that budgets are often exceeded.  If that becomes the case for us, then this deficit number needs to zero.  Thus we have to write a sightly more complicated calculation:

Since we've already walked down the path of going over budget, we need to calculate that piece as well.  Similar to the calculation above, there is no excess if the actual/budget is less than 100%, so we'll turn the alligator mouth around to make this zero when there's a deficit.  Here's the other thing - this still has to work if you go over 200% of target.  So with all that in mind here's how we calculated the excess:

Int() basically acts a floor function, and returns only the amount above x00%.  Lastly, we need to create one more field to be a place holder.  I always go will a field called "One" which is equal to (you guessed it) 1.

Ok, let's get to vizzing.  Grab the last field we created, "One" and throw it on columns twice.  By default, it's going to show the SUM(One) so right click and change each to be a dimension. Next, right click the second of the two and make it a dual axis.  You should now have something that looks like this:

The first "One" is going to become a pie chart.  I know, it's painful for me too, but that's what it takes to make a donut chart.  So select it and change the mark type to pie.  Now we're going to take Measure Names and put it on the Filters shelf, and select the three measure you created first (for me this is: Margin - Percent of Budget, Margin - Percent of Budget Remained, and Margin - Percent Extra).  Also add Measure Names to the color shelf, and finish this part off by adding Measure Values to the Angle Shelf.  Should now look something like this:

Now for the second "One" - this need to become a circle mark type.  Once you've switched that, bring the "Percent of Budget" calculation in and put it on the Label Shelf and change the alignment of it from auto-auto to center-center (all this time I've been it the Text Shelf - learn something new every day).  Click on size and decrease it until it's about half the size of the pie chart.  Click on color and turn it white.  Should look something like this:

Clean up the formatting (Uncheck show header on both "One"s, get rid of the zero line, click color shelf and add a borders to both the circle and the pie chart, etc....).  Last thing I'll do is put "Product" up there on the Columns Shelf, so I can see examples where there's both excess and deficit.  And...

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Ta-da!  Donut charts!  That was a fun one.  Makes me hungry.  Thanks always for hanging out! 

Nelson