Context is important in data visualization. For every chart you create, consider asking yourself "compared to what?" 

In this tip, I show you how to add context to a simple bar chart by showing whether a value is above or below the average.

UPDATE – 10-Apr-3014:
I received some feedback from both Jonathan Drummey and Joe Mako about this blog post and some of its inaccuracies.  There are a couple of key notes:

1. My intent was to show how you can compare the 7-day averages of two time periods. In this example, I’m calling this a Year over Year calculation, but really it’s a comparison versus 365 days ago. Small, but important distinction.
2. The Superstore Sales data set has days that are missing, so unless you turn on domain padding, you won’t be comparing the prior 365 days that you’re expecting to compare. I’ve updated the post below.
3. Like most things in Tableau, there are many ways to solve the same problem. Joe pointed out some things he would have done differently with the calculations. While neither of us is wrong, note that you should always look for ways to be more efficient.

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April is #TableauTipsMonth, so I thought I would pick something from my backlog and get to it. Today I’ll be writing about a fairly typical scenario:

1. Your data is cyclical through the week
2. Daily numbers vary wildly, so smoothing is necessary
3. You need to compare to the same period from the prior year

This can be handled pretty easily and understandably with a few table calculations. To start, let’s look at daily sales for 2013 in the Superstore Sales data set to illustrate the wild nature of daily data that many of us see.

This data set looks like it might be cyclical, so let’s apply a 7-day average calculation to it. We could do this via the quick table calculations on the pill, but we will want this calculation for later. Right-click on Sales in the Measures pane and choose Create Calculated field. Build this calculation:

Here’s what the chart looks like if I filter Order Date to 2013 on the Filter shelf (I’ve focused in on January):

Since I have filtered the year of Order Date to 2013 by dragging the Order Date pill onto the Filters shelf, Tableau first ran a query that returns only results for 2013 and then it will do the 7-day average calculation. We don’t want that because the first six days of 2013 will be wrong. They are wrong because they don’t contain a full 7-day date range.
To fix this, we need to change the calculation. Notice that I’m not passing a filter to Order Date inside the calculation. By passing the filter in the table calc, Tableau will return all of the data for all years, then the table calc will running after the database query, which will then provide the full date range for the 7-day average to calculate correctly.

And this is what the chart looks like if filter the data via the table calculation (of course you have to remove Order Date from the Filters shelf):

Quite a different story. The lesson here is that you need to understand how Tableau is filtering the data. Anything on the Filters shelf will filter the data before any table calcs are performed.
One thing to note, though, is that this data set does not include all dates. Therefore, we need to turn domain padding on by right clicking on the Order Date pill and checking the “Show Missing Values” option. Notice how there are now gaps in the line; that’s because Tableau is filling in the missing dates for us.

So that covers our 7-day average. To calculate the 7-day average for the same 7-day period 365 days ago, we only have to make a simple adjustment to the 7-day average calculation we’ve already created.

The only change is the start and end period inside the WINDOW_AVG function. Now drag the new measure onto the same axis as the 7-day average.

So now we can see how the 7-day average on an day compared to the 7-day average 365 days ago. Perfect!
Now that we have these two table calcs, we can easily compute the change between the two dates periods with another calculated field:

Drag this new measure onto the Rows shelf. Clean it up a bit, and we now have a nice view that answers a simple question: How are sales performing compared to last year?

Notice that I used an orange-blue color palette for the comparisons to the prior 365 day values. This is a good practice to employ because the colors corresponding with the colors for each line. If the bar is orange, then last year performed better and vice versa.
Download the workbook used in this example here.