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 Combined Legends vs Separate Legends

What are combined legends?

When creating some visualizations, it is possible to combine information about different data object properties into a single legend. This simplifies the graph and reduces the amount of legends that need to be displayed to convey the same amount of information. Consider the following example:

In this graph, both the Color and the Size of the symbols has been defined by the same categorical variable Sex. For this graph:

The size of symbols for "Female" symbols is 10, the size of "Male" symbols is 5

The color of symbols for "Female" symbols is blue, the color of "Male" symbols is red

Using this information, Prism created a legend with two entries:

1.Female: size 10 blue symbols

2.Male: size 5 red symbols

In theory, two separate legends could have been created for this graph (and it's possible to create this visualization in Prism; see How to separate legends). Here's what that graph would look like:

Now, there are two separate legends. In the first, the legend shows that the blue symbols are for Female, and the red symbols are for Male. In the second, the legend shows that Male symbols are size 5 while Female symbols are size 10. These two graphs actually convey the exact same information, but the first is arguably easier to interpret due to the fact that it uses a combined legend.

How does Prism determine when to combine legends?

Prism will automatically determine if two legends can be combined. Here are a couple of general rules:

If a color legend is defined by a continuous variable, it cannot be combined with other legends

If two legends are defined by different variables, they cannot be combined with other legends

This is not an exhaustive list of rules for combining legends. But fortunately, you typically don't have to think about these rules. Prism will assess your data and your variable assignments and determine for you if any legends can be combined.

How to separate legends

What if you don't want a specific legend to be combined with others on the graph? In this case, you can check the box under "Separate Legend" for the given legend. This will instruct Prism to NOT combine this legend with any others.

Note that if you check the "Separate Legend" box for a legend that cannot be combined with any others (due to the variable assignments on the graph), no visual change will occur (a legend that cannot be combined will already be separated, so telling Prism explicitly to separate it will have no effect on the appearance of the graph).