_autolev(self, N)
The target number of levels, N, is used only when the scale is not log and default locator is used.
We need two more levels for filled contours than for line contours, because for the latter we need to specify the lower and upper boundary of each range. For example, a single contour boundary, say at z = 0, requires only one contour line, but two filled regions, and therefore three levels to provide boundaries for both regions.
Select contour levels to span the data.
Hover to see nodes names; edges to Self not shown, Caped at 50 nodes.
Using a canvas is more power efficient and can get hundred of nodes ; but does not allow hyperlinks; , arrows or text (beyond on hover)
SVG is more flexible but power hungry; and does not scale well to 50 + nodes.
All aboves nodes referred to, (or are referred from) current nodes; Edges from Self to other have been omitted (or all nodes would be connected to the central node "self" which is not useful). Nodes are colored by the library they belong to, and scaled with the number of references pointing them