If the question asks how to show e.g. min, max and avg values of a graph in a legend of the panel, here is the answer: While editing a panel in Grafana, there is a bunch of tabs about in the middle of the display (including General, Metrics, Axes, Legend and so on...). From there you should choose the Legend tab where, under the Values title, you can select what is shown along with the legend of your graph. Here is a link to Grafana's Graph Panel Legend documentation where you can read more.
If the question is asking how to fetch those values for some other external use, then I don't know.