Instructions:

LMB click: -on background creates vertex -on vertex selects/deselects it D: -turns Dijkstra mode ON/OFF

While Dijkstra mode is running selected vertexes appear yellow instead of blue. To create edge select two vertexes. If edge already exists it gets deleted. To run Dijkstra's algorithm turn Dijkstra mode on and select two vertexes(after it finishes press D again).