How to Create a Gradient Area Chart in Tableau (Kizley Benedict)

 

This week, I’m excited to welcome Kizley Benedict as a guest author. If you’re not already following Kizley on Tableau Public, stop right now and go follow him—I promise you won’t regret it. He has a rare talent for creating well-designed and insightful data visualizations. When Kevin and I first saw the lovely Shaded Area Chart on Tableau Public (see below), we immediately reached out to see if he would be willing to write about it. While there are some similarities to what Kevin and I did with gradients in the past, his method is easier and cleaner than what we’ve done, so we’re excited to have him share it.

 

Kizley is a Data Visualization Consultant for Intelligent Cricket in Delhi, India. He has worked in the field of Data Analytics for the past eight years and has been using Tableau since 2016. As noted above, he is a very popular author on Tableau Public. You can follow him on Twitter @kizley and LinkedIn. Thanks so much for sharing your knowledge with us today, Kizley!

 

Inspiration

If you have ever googled the name of a company stock, chances are that you would have come across a trend line chart that shows the stocks historic performance. For example, a quick search query of ‘Tesla stock’ displays the below google result:

 

Source: google.com

 

What caught my eye was the light green gradient visible below the line. Does it add any value to the overall graphic? Not really, but it looks quite aesthetic and I thought it would be an interesting challenge to try to build something similar in Tableau.

 

Here is what I managed to create in Tableau. Click on the image to view the interactive version.

 

  

 

Note: The above visualization was built using map layers and it everything was built using a single sheet. However, in this tutorial we will be using the dual axis method as it’s a bit simpler and more straightforward. The underlying concept is essentially the same for both.

 

Use Case

The gradient area chart can be used to display any measure over time provided all values are on the same side of the horizontal axis (all positive or all negative). E.g., Weekly Sales Trend. Conversely, a metric like profit over time would likely have both positive and negative values and hence would not be a suitable use case.

I would also not recommend this method where dashboard performance is required to be optimal or if the workbook contains large volumes of data. While the calculations used in the method are relatively simple, adding background images to the views might have a slight performance implication.

 

Approach

Since the gradient is mostly an aesthetic choice and does not add much insight, I wanted to avoid using complex calculations or other performance-intensive operations like data densification. The next logical option was to use a background image as a gradient. However, the tricky part is that the gradient should only be visible below the line so the part of the image above the line would need to be hidden.

 

 

If only I could stack a reversed area chart above the line and color it white that would do the trick!

 

Background Images

Before we start with Tableau, we would first need to create a couple of background images which would serve as the gradient for our area chart. I used Figma to create the gradients but you could also easily create these in PowerPoint or any other image editing tool.

 

Below are the two gradient images in .svg format I will be using for this tutorial (.png works as well). The blue gradient will be displayed in case of positive values and red in case of negative values. We want the color intensity to correspond to the value of the measure used, i.e., the intensity is highest at the top and the transparency increases as we move downwards.

 

Note: If you'd like to use these yourself, you can download them here.

Now, let’s open Tableau and start building the view. We will be using the Sample-Superstore dataset to:

1)  Show a YTD weekly sales trend as a gradient area chart.

2) For a selected Sub-Category, show a red gradient in case of negative YoY variance and blue gradient in case of positive YoY variance

 

Part 1: Create Calculations for the X and Y Axes

 

1) Create a calculated field called Week Date using Order Date.

 

Week Date

// Get the week.

WEEK([Order Date])

 

Since our X-axis is required to display YTD values, it needs to be dynamic. This presents a challenge while adding the background images which requires us to input fixed start and end points for the axis. The way around this issue is to normalize our date values so that it lies between a fixed range such as 0 to 1.

 

2) Create the following calculations to normalize the date values:

 

Min Date
// Minimum Date
WINDOW_MIN(MIN([Week Date]))
 
Max Date
// Maximum Date

WINDOW_MAX(MAX([Week Date]))

Date Normalized

// Continuous measure in the range of [0,1]. Min date will be 0 and Max will be 1.

// We can then use this field to set the limits of the background image.

(MIN([Week Date]) - [Min Date])/([Max Date] - [Min Date])

 

Note: As mentioned before, the above steps are only required for dynamic axis values. If your date axis is fixed, you can skip the above steps and directly input the start and end dates while positioning the background image. Also, if the frequency is monthly then you can input the start and end points as 1 and 12 respectively without having to normalize the axis.

 

3) Finally, we need to normalize the Sales values by the same logic. So, let's create the following calculations:

 

Min Sales

// Minimum sales amount.

WINDOW_MIN(SUM([Sales]))

 

Max Sales

// Maximum sales amount.

WINDOW_MAX(SUM([Sales]))

Sales Normalized

// Similar to ‘Date Normalized’, this restricts the range of the Sales values between 0 and 1...

// ...giving us a fixed start and end point for the Y-axis.

(SUM([Sales])-[Min Sales])/([Max Sales] - [Min Sales])

Sales Reversed

// This will be used to hide the area above the trend line.

(1-[Sales Normalized])

 

Note: The above will truncate the Y axis, which is often not something you want to do. If you wish to avoid truncating the axis, then simply set the Min Sales calculated field to equal zero.

 

Part 2: Creating the View

 

1) In a new sheet drag Order Date to the filters shelf as a ‘Relative Date’ field. Click on ‘Years’ and select ‘Year to Date’. Make sure the anchor is set as relative to ‘Today’. This will be the YTD filter.

 

 

2) Add Week Date field to ‘Detail’ on the Marks Card. Click on the drop down and change the mark type to ‘Line’

3) Drop Date Normalized in the columns shelf as a continuous field and compute it using Week Date.

4) Add Sales Normalized to rows and compute it using Week Date.

 

The view should look something like this, with both axes in the range of 0 to 1:

 

 

5) Add Sales Reversed to rows next to Sales Normalized. Compute it using Week Date.

6) Edit the axes for both Sales Normalized and Sales Reversed setting them to go from -0.05 to 1.05. This will ensure that the two axes are aligned.

7) Under Scale select the ‘Reversed’ checkbox.

8) In the Marks dropdown for Sales Reversed select ‘Area’. Change the color of the area chart to the background color of the sheet. In our case it is white.

Set color opacity to 100%.

9) Click on the Sales Reversed pill in the rows and select Dual Axis. Note: If Tableau adds Measure Names to the color card, then go to the All marks card and remove it.

10) Right-click on the Sales Normalized axis > Move marks to front

 

After executing the above steps, you should be seeing the below view:

 

 

The part of the sheet above the line is covered by the white area chart. The area below the line is where we want the background image to be visible.

 

Part 3: Adding the Background Image

 

1) Go to Map -> Background Images -> Sample-Superstore -> Add Image

2) In the dialog box which appears, under the ‘Image’ tab add the location of the blue gradient image.

3) In the ‘X Field’ dropdown select Date Normalized. Set left to 0 and right to 1.

4) In the ‘Y Field’ dropdown select Sales Normalized. Set bottom to 0 and top to 1.

 

 

5) Click on ‘Options’ and make sure both checkboxes under ‘Image Options’ are unchecked. Click OK and close the dialog box.

         

If all goes well you should see a gradient area chart as shown below:

 

 

Since the Y-axis does not show the actual Sales values, we are going to hide it and instead, add reference lines and labels to show the maximum and minimum values in the view.

6) Drag SUM(Sales) to the Sales Normalized marks card and drop it on Label. Click on Label and select ‘Min/Max’ under Marks to Label.

7) From the Analytics pane drag ‘Reference Line’ into the view for the Sales Normalized axis. Use the ‘Band’ option to add minimum to maximum reference lines.

8) Right click on the Sales Normalized axis and uncheck ‘Show Header’,

9) Turn off row/column dividers and zero lines for both axes.

 

We finally have a gradient area chart with labels showing the minimum and maximum Sales values.

 

 

Part 4: Making the Gradient Dynamic

 

The next challenge is to make the gradient colors change based on a measure value. To accomplish this, we are going to create a YoY sales variance calculation and for a selected Sub-Category, display a blue gradient for positive variance and red for negative variance.

 

1) Drag the Sub-Category field to filters and select ‘Accessories’. Click on the pill and ‘Add to Context’.

2) Create the following calculated fields:

 

CY Sales

// Sales for current year only.

IF YEAR([Order Date]) = YEAR(TODAY()) THEN [Sales] END

 

PY Sales

// Sales for prior year only.

IF YEAR([Order Date]) = YEAR(TODAY())-1 THEN [Sales] END

 

YoY Sales Variance

// Variance between current year and last year.

(SUM([CY Sales]) - SUM([PY Sales]))/SUM([CY Sales])

 

3) Create another calculated field called Image Filter: This will be used to switch the background image to blue or red depending on the YoY sales variance

 

Image Filter

// We'll use this to switch the background image to blue...

// ...or red depending on the YoY sales variance.

{ FIXED [Sub-Category]: [YoY Sales Variance] } >=0

 

4) In the Sales Normalized marks card drop the Image Filter field on Color. Assign red color for False and blue for True.

 

In order to make the images dynamic we are going to use the Image Filter field and set a condition to show or hide each image.

 

5) Edit the previously added blue gradient image. Click on the ‘Options’ tab in the dialog box. Click ‘Add’ and choose Image Filter from the list of fields. Set the value to True.

 

 

6) Next, we are going to add the red gradient as a background image with exactly the same inputs as the blue gradient. As with the first image, make sure both the checkboxes under the ‘Options’ tab are unselected.

 

 

7) Click on the ‘Options’ tab in the dialog box. Click ‘Add’ and choose ‘Image Filter’ from the list of fields. Set the value to False.

 

8) As a final step we are going to add the actual Week Dates on the Y- axis in a separate sheet and place it below the area chart on the dashboard.

 

And we are done! Our area chart is now dynamic showing a red gradient for negative YoY variance and a blue gradient for positive YoY variance.

 

Click on the image below to view the interactive version.

 

 

I hope you had fun following along this tutorial. I’m excited to see what others come up with using this concept. If you have any questions, please feel free to reach out to me on Twitter (@kizley) or Linkedin (linkedin.com/in/kizleyb/). Thanks for reading!

 

Kizley Benedict

September 6, 2022