Graph has dots with a horizontal lines attached to each dot

I have a question about a graph that looks different than any that I have seen before. The graph has ten dots that could be connected to create a slope rising from left to right. Each dot on the graph also has a single horizontal line going from the dot to one measurement to the left. A question asked about the slope between x = -2 and x = -3. at those two points, Y is -3 and -2 respectively so the slope is 1 but the answer says the slope is 0. It must have something to do with those lines but I do not know what they mean.

8th grade math