A reader emailed me this chart of GDP, with actual values through late 2008 and projected values for the next two years. To illustrate the uncertainty in the predictions, colored bands were drawn alongside the solid line prediction. The bands become lighter as the distance from the prediction line increases. The reader wondered how to create this effect in a chart.

The chart comes from page 34 of A Preliminary Analysis of the President’s Budget and an Update of CBO’s Budget and Economic Outlook, published in March 2009 by the Congressional Budget Office.

I’ve manually digitized the data into this tall thin tower of numbers.

Here is the data plotted in a line chart.

To capture an assumed uncertainty in the numbers, I’ve set up the following table, in which I’ve assumes a minimum and maximum for January 2009 onward, and I’ve inserted intermediate columns a, b, c, and d.

When I add these columns of data as additional lines in the chart, I get the following. The lightest gray lines are difficult to resolve, but then, they are the most uncertain. This is fine, but it doesn’t have the filled area effect.

To achieve filled regions, I will use a stacked area chart type. In an adjacent range I’ve added the following table. The dates are the same, as are the minimum and midline predictions. The rest of the data are the calculated differences between each line and the previous, which capture the height of each colored band.

Using the series from Min through Max I’ve created a stacked area chart. There are my colored bands.

I changed the bands to appropriate shades of gray, and the minimum series is not filled at all. You can use any color scheme you like, and as many different colors and data sets as you need. I used shades of gray because these were already plentiful in my custom color palette.

To get the solid line for the midline predictions, add the midline data as a new series. Initially it’s plotted as another area stacked upon the rest.

Convert the added series to a line chart type, and format it appropriately (I’ve used dark gray).

That was pretty easy, and it’s got to be one of the shortest tutorials I’ve ever written.

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