See attached screenshots. This is using the stock default discovery filesystem rules for graph creations.
If there is a wide separation between total disk space and free disk space, the Total disk space line is not displayed on the graph.
If the separation is minimal between free and total, then both lines are displayed as expected. I don't know what the threshold is.
This occurs on all platforms where this discovery process is used

Screenshots:
no_total.jpg: graph showing there is no "Total" line
no_total_graphs.jpg: Configuration of stock graph prototype
no_total_item.jpg: Configuration of discovery rule