1. Creating a symbol map
We’ve connected to the Gapminder data source and we’re interested in learning which countries had the most cases of lung cancer. Before we dive into the process though, let’s take a quick look at the data pane. Here we have one geocoded dimension. This field is marked with a globe icon. When we drag Country from Dimensions into the view, two things happen. The first is that Tableau automatically adds the Country field to detail on the Marks card. The second is that Tableau generates the latitude and longitude fields on Columns and Rows since the Country field was geocoded for us. Before starting, let’s filter the data on the latest year in our dataset, 2008
Now that we’ve generated a map, let’s see which countries had the most lung cancer cases. First, we’ll drag Lung Cancer from Measures to Size on the Marks card. We can increase the size of the circles, so it’s easier to see which countries had the most cases. A symbol map is perfect here because the circles in our view standardize how the data is shown and you don’t get a biased view of lung cancer cases based on the relative size of the countries. Instead, you’re focussing on the size of each circle within each country. It’s important to note that Tableau will assign the smallest circle size to the lowest value and the largest circle size to the highest value. Everything in between is represented by a proportional size based on the actual lung cancer cases. So it’s not a good idea to put a measure with potentially negative values on Size because it’s not immediately clear whether a mark with a negative number is positive or negative.
Now let’s see how the countries compare with regards to their population growth rate, meaning the percentage change in population compared to the previous year. We can drag population Growth to Color on the Marks card. Notice that the default aggregation is SUM. Let’s change this to average. Then, we can click Color on the Marks card and add a black border around the circles, and remove the halo. This makes the circles stand out more from the grey map image. We can see by the orange circles that some countries’ population declined from 2007 to 2008. Note that although measures with negative values are not recommended on Size, they work really well on Color. You can adjust the background map settings too. Select the Map menu, then click Map Layers. The Map Layers pane will appear on the left panel. We can unselect Land Cover and select Coast Line. You can see how the map view changes with each selection. Looks good right? Symbol maps are great if you want to show two measures on the same map. Both circle size and color carry information we can use to make decisions. Okay, let’s practice.
2. Let's practice!