Is there a way to tell Mathematica to use plain lines for contour lines associated with positive level values and dashed otherwise ?
Can't seem to find anything in the help documentation.
Thanks for the help.
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- 1$\begingroup$ Posible duplicate: https://mathematica.stackexchange.com $\endgroup$José Antonio Díaz Navas– José Antonio Díaz Navas2018-01-29 23:43:29 +00:00Commented Jan 29, 2018 at 23:43
- $\begingroup$ That is right on point. Thanks. $\endgroup$jrekier– jrekier2018-01-30 09:26:10 +00:00Commented Jan 30, 2018 at 9:26
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4 Answers
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Maybe this? :
ContourPlot[Evaluate@((x*y == #) & /@ Range[-8, 8, 1]), {x, -3, 3}, {y, -3, 3}, ContourStyle -> (If[# >= 0, {Directive[Red]}, {Directive[Blue, Dashed]}] & /@ Range[-8, 8, 1])] 
answered Jan 29, 2018 at 23:42
José Antonio Díaz Navas
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1 f[x_, y_] = x*y; Show[ ContourPlot[f[x, y], {x, -3, 3}, {y, -3, 3}, Contours -> Range[1, 8], ContourShading -> None, ContourStyle -> Red], ContourPlot[f[x, y], {x, -3, 3}, {y, -3, 3}, Contours -> Range[-8, -1], ContourShading -> None, ContourStyle -> Directive[Blue, Dashed]], ContourPlot[f[x, y], {x, -3, 3}, {y, -3, 3}, Contours -> {0}, ContourShading -> None, ContourStyle -> Directive[Lighter[Gray], AbsoluteThickness[1]]]] 
- $\begingroup$ I must be missing something. Why would this answer be downvoted? Bob's answer also anticipates a zero contour. $\endgroup$JimB– JimB2018-01-29 23:12:26 +00:00Commented Jan 29, 2018 at 23:12
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This hack-ish method relies on ContourPlot[] generating contours with the $z$-value being stored in a Tooltip[]:
ContourPlot[x y, {x, -3, 3}, {y, -3, 3}, Contours -> 12, ContourShading -> None, ContourStyle -> {}] /. Tooltip[prims_, val_] :> Tooltip[Prepend[prims, Switch[Sign[val], -1, Directive[Blue, Dashed], 0, Opacity[1/2, Gray], 1, Red]], val] 
answered Apr 11, 2018 at 12:58
J. M.'s missing motivation
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Yes, you can create two separate plots (one for positive, one for negative), and combine them:
Show[{ContourPlot[{Abs[Sin[x] Sin[y]] == 0.5, Abs[Cos[x] Cos[y]] == 0.5}, {x, -3, 3}, {y, -3, 0}, ContourStyle -> Dashed], ContourPlot[{Abs[Sin[x] Sin[y]] == 0.5, Abs[Cos[x] Cos[y]] == 0.5}, {x, -3, 3}, {y, 0, 3}]}, PlotRange -> All] 
