Contour Plots
Contour lines connect points of equal value, showing a 2D scalar field's levels, gradients, and structure with precision.
Lines of equal value
A contour plot draws curves along which a 2D scalar field is constant, exactly like elevation contours on a topographic map. Where lines bunch, the field changes fast; where they spread, it is flat. Contours give precise level information that a smooth color wash alone cannot.
Filled and line contours
Line contours mark specific levels and read gradients well. Filled contours color the bands between levels, combining the value cue of a heatmap with the discreteness of contours. Using both, thin lines over a filled map, gives level precision and overall shape at once.
- Even level spacing makes gradient steepness readable from line density.
- Labeled levels remove ambiguity about which line is which value.
- A perceptual fill palette keeps the color honest.
Choosing levels
Level selection shapes the story. Too few levels hide structure; too many clutter. Levels tied to physical thresholds (a boundary value, a critical pressure) make the plot interpretable. Logarithmic levels suit fields spanning many orders of magnitude.
Relation to isosurfaces
A contour is the 2D case of an isosurface; the 3D generalization uses marching cubes (see Isosurfaces). Contours on stacked slices are a lightweight way to convey a 3D field.
Kronos use
Poloidal flux is shown as nested contours in equilibrium cross-sections, where the closed contours are the flux surfaces and their spacing conveys the field gradient.