fix: state: last to use value from last Graph coordinate

[whisdol]

Currently, state: last breaks rendering with bar charts (#1248) because points are not calculated for bar charts.
This fix accesses the value of the last coordinate from the Graph instead, which is exactly what I want to have displayed when using this option.
To my understanding (and testing), using the last coord is a simpler and more reliable fix than always calculating points (ref #1075), because the last point for a grouped bar chart is not the value of the last coordinate (the last point is actually the average of the last two states, so a completely different value). (Surprisingly, the same is true for grouped line graphs, where the point is selectable and shows this incorrect value as the hover state, but that may be a different issue.)