Add suggestion from review

Author: Zetison

docs/make.jl

@@ -52,6 +52,7 @@ makedocs(;
             "Customize descriptive_names"=>"how-to/customize-descriptive_names.md",
             "Improve performance"=>"how-to/improve-performance.md",
             "Use custom backgroun map"=>"how-to/use-custom-background-map.md",
+            "Add additional plots"=>"how-to/add_additional_plots.md",
         ],
         "Library" => Any[
             "Public"=>"library/public.md",

docs/src/how-to/add_additional_plots.md

@@ -0,0 +1,49 @@
+# [Add Additional Plots](@id how_to-add_additional_plots)
+
+Use the `additional_plots` keyword in the [`GUI(::Case; kwargs...)`](@ref) constructor to add external time series (or other indexed series) in the results axis, alongside model data.
+
+The `additional_plots` keyword argument accepts a vector of dictionaries. Each dictionary defines one additional plot source:
+
+- `"data"` (**required**):  
+  - a `DataFrame`, or  
+  - a path to a CSV file.
+- `"normalize"` (*optional*, default `false`):  
+  If `true`, the series is scaled before plotting, which is useful for shape comparisons with model results.
+
+The time index of the additional data must match the time index of the model results. The time index column must be named one of the following `t`, `sp`, `rp`, `osc`, `op`. If this column is missing, an `OperationalProfile` will be created for the additional data (where the time index is inferred based on the row order to match the time structure of the model case), and a warning will be issued.
+
+## Example
+
+Assuming you have a CSV file with additional time series data located at `path/to/your/data.csv`, for example with content
+
+```csv
+sp,val1,val2
+sp1,0.5,0.6
+sp2,0.6,0.7
+sp3,0.7,0.8
+sp4,0.8,0.9
+```
+
+you can load it into the GUI as follows:
+
+```julia
+using EnergyModelsGUI
+
+gui = GUI(case; additional_plots=[Dict("data"=>"path/to/your/data.csv", "normalize"=>true)])
+```
+
+Adding a data frame directly is also possible, for example (assuming the time index is `sp1-t1`, `sp1-t2`, ..., `sp1-t10`):
+
+```julia
+using EnergyModelsGUI, DataFrames
+
+t = "sp1-t" .* string.(1:10)
+additional_data = DataFrame(t=t, series1=rand(10), series2=rand(10))
+gui = GUI(case; additional_plots=[
+        Dict("data"=>"path/to/your/data.csv", "normalize"=>true), 
+        Dict("data"=>additional_data),
+    ]
+)
+```
+
+After loading the GUI the additional series are available in the **Available data** menu (make sure not to have any element selected), and can be selected for plotting alongside model results.