Adding different strings to legend title in QGIS layer composer depending on which layer is active

Question

My QGIS project includes three layers representing catchment areas for playgrounds. Depending on the age of the children, the catchment areas are either 200 m (catch_area_200m.shp), 400 m (catch_area_400m.shp), or 1000 m (catch_area_1000m.shp).

Within the print composer always just one catchment area is visible. Depending on which of the three layers is active, I want to have this information in the title of the legend.

What I basically need is some sort of conditional clause, checking which layer is activated and then printing a string with the appropriate distance, e.g. '400 Meters', to the title of the legend.

How can this be realised?

Answer 1

You can solve it using the function is_layer_visible in an expression.

In your Legend > Item proprieties select the Data Defined Override on the right of the title and, then, Edit...

Here you can use this expression

case 
when is_layer_visible('poly') then concat ('100m')
when  is_layer_visible('poly 2') then concat ('200m')
else ''
end

Change the name on the layers (here poly and poly 2) with yours.

Now, the legend title will be depending on the activated layer.