In Inkscape when I apply a dash stroke, then set a mid marker shape, it does not show anything.
To get mid-markers to show, you need to have one or more nodes in the middle of the path, or as the Inkscape developers say*
Mid Markers are drawn on every node of a path or shape except the first and last nodes
If you already drew a line with only two nodes (a start and end node), and you want to add one exactly in the middle, select the segment with the Edit Nodes tool, then hit the Insert Nodes button in the tool controls bar along the top.
*Note that if you mouse over the mid-marker, the Tool Tip tells you this
The term “Mid Marker” is a little confusing, because
it doesn't mean "in the middle of" or “in the center of”, but rather "between", where
- “between” means “between the first and the last node, not including these nodes”,
it doesn't mean “somewhere” between, but “at all nodes” between.
So the mid marker says something as “I am a node, but neither the first, nor the last one”.
As the mid marker doesn't mark the first and the last nodes, you need at least one other node (i.e. at least 3 nodes altogether) in your path to see (at least) one mid marker.