I would like a graph which reflects the true position of inventory/goods on
hand. Currently we have a line graph with quantity on the x-axis and dates on
the y-axis. The problem is that the if you have 30 units on the 1st May and
then 50 units come into store on the 15th May the line joining the two points
is a slope which incorrectly shows stock increasing consistently from the 1st
of May until the 15th of May. The correct graph would show a line of 30 units
along the period 1st to 14th May and then jump to 80 units on the 15th May.
Is there a graph type which reflects this situation?

Thank you for your assistance.