If you enable the value mapping for an item this is visible in the overview. When the same item is graphed (by default as the actual value is a number), the mapped value is not visible in the graph.
This means that you have no clue which the specific value means.
For example I'm monitoring heartbeat machines which have a status active or passive. These values are stored as 0 and 1 and mapped to standby and active. On the overview page this is visible, but the graph only shows 0 and 1 and no direct link to which theses values mean.