When you click on the button itself, it opens a separate display, with some additional options and information about the available timeseries.
Hide whole forecasts (since 2020.02)
Since 2020.02 it is possible to hide all time series belonging to the same forecast.
When time series are selected, with the dropdown button or ctrl+alt+shift+X all time series sharing a forecast with one of the selected forecasts will be hidden.
In the image belong forecast  has been made hidden.
In the search and select forecasts there will be a postfix stating (hidden) behind their forecast time.
In the image below forecast 1 and 8 have been hidden.
All hidden forecasts can be made visible again by the "Show all hidden forecasts" dropdown menu item or with ctrl+alt+shift+Z.
With this button historical events can be activated (provided that they are pre-configured). They will be displayed in the same graph to enable a comparison with the actual timeseries. Further explanation is given below.