Slope graph compares the trend of one field against two groups. This diagram only displays an A to B relation; the points in between are not relevant. This diagram is fairly similar to a line multiples with an emphasis on the start and end points.
A slope graph accepts one X-Axis, one Y-Axis and up to two Group Fields.
If you select numeric data for the Y-Axis field, please remember to select an Expression by clicking on the field first and then choosing from the drop down.