WPSolr isn’t just a WordPress search plugin. It can also add facets.
This easy to understand guide should outline how you could do that.
Setup WPSolr WordPress search
Learn how to setup WordPress search using WPSolr.
Create facets using WPSolr
Click on the WPSolr “2.4 Facets” tab in the WPSolr dashboard.
Select the data types to include as facets (filters).
Click on the “Save Options” button to save the facets.
This will reload the page. Each one of the selected facets will expand with many options to choose from.
Select the label for this specific facet. This will appear on top of the facet from a user point of view.
For each one of the facets, you can select the layout. Only 2 are available in WPSolr Free : Check boxes and select boxes. If you need more facets you should upgrade to pro.
Then once you have selected your layout you can toggle options. For example, toggle hierarchy.
Once you have configured all your facets, you can save.
You can then choose where to place the newly created WPSolr facets by opening the WordPress widgets tab.
Add the “WPSolr Facets” to the sidebar.
You can now view the facets in the front-end.