contour plot with positive/negative values - how to set the line style for (negative) dashed values

[Alessandro Mandelli]

Etienne wrote:

> here are the options I put:
> 'set ccolor 1'
> 'set gxout contour'
> 'set cthick 6'

Use cmin 0 for plotting with solid line (set line color# <style>
<thickness> with style = 1)
Use cmax 0 for plotting with dashed or other color line (set line color#
<style> <thickness> with style = 3 or 5)