In cases it can be handy to show values while mousing over a graph.
I found this example, where you seem to drag a red indicator along the graph, that also shows the exact time for the readings. The moused-over readings could just be shown in an additional column next to last, min, avg and max. That should be space-saving and not mask the actual graph.
From a technical point of view, the data could be transmitted along after generating the graph. It's already retrieved at that time and dynamically polling for it is certainly uneconomic.