Update explicit_interface.md

[ci skip]

Author: PhilipVinc

docs/src/explicit_interface.md

@@ -38,3 +38,19 @@ log_audios
 ```@docs
 log_embeddings
 ```
+
+## Custom Scalars plugin
+See [TensorBoard Custom Scalar page](https://github.com/tensorflow/tensorboard/tree/master/tensorboard/plugins/custom_scalar).
+
+For example, to combine in the same plot panel the two curves logged under tags `"Curve/1"` and `"Curve/2"` you can run once the command:
+```julia
+layout = Dict("Cat" => Dict("Curve" => ("Multiline", ["Curve/1", "Curve/2"])))
+
+log_custom_scalar(lg, layout)
+
+```
+
+See also the documentation below
+```@docs
+log_custom_scalars
+```