Inventory Planner collects your historical stock starting from the moment of the installation. You can it for variants, products, categories and vendors in Inventory KPIs report.
Click on Inventory KPIs -> Total and then use the bulleted list icon in the Details column
then click on the Stock & Sales History section
to visually see it on the graph
Download it in different formats using the hamburger icon
You can also add the historical stock information to the report. Enable Opening/Closing Stock columns in the report settings. These columns show you the stock at the beginning and at the end of the selected time period.