contour_plot bug

[sagetrac-mabshoff]

Kate reported in https://groups.google.com/group/sage-support/browse_thread/thread/843c9452036e9608/d5c4e6a830a66327#d5c4e6a830a66327

In 2.10.1,

  sage:  R.<x,y> = PolynomialRing(QQ,2)
  sage:  contour_plot(y-1,(-10,10),
(-10,10),fill=False,contours=1,plot_points=100)

plots the line y = -9 rather than the line y=1

  sage:  contour_plot(-y+1,(-10,10),
(-10,10),fill=False,contours=1,plot_points=100)

correctly plots y = 1.

Kate

David Joyner could confirm the bug:

sage: contour_plot(y-1,(-10,10),(-10,10),fill=False,contours=2,plot_points=100)
sage: contour_plot(y-1,(-10,10),(-10,10),fill=False,contours=1,plot_points=100) 

Cheers,

Michael

Component: graphics

Keywords: contour, plot

Issue created by migration from https://trac.sagemath.org/ticket/2243

[sagetrac-anakha]

comment:1

I think contour_plot is misused here. 'contours=1' tells contour_plot to draw one contour without specifying its value. If you want to really draw the portion where the function equals 1 and nothing else, use 'contours=[1]'.

[mwhansen]

comment:2

I think Arnaud's comment is the correct one. If you look at the (current) documentation for contour_plot, you'll see

            contours     -- integer or list of numbers (default: None):
                            If a list of numbers is given, then this specifies
                            the contour levels to use.  If an integer is given,
                            then this many contour lines are used, but the
                            exact levels are determined automatically.
                            If None is passed (or the option is not given),
                            then the number of contour lines is determined
                            automatically, and is usually about 5.