What is the problem with my plot? I want to get a disk shape.
Plot[Table[-x + k, {k, -2, 2, .1}], {x, -5 ,5},
RegionFunction -> Function[{x, y}, (x^2 + y^2) < 1]]
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2 Answers
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I think this is a bug. Here is a test with a single straight line and a RegionFunction that differs in its threshold only by .007:
Table[
Plot[-x, {x, -5, 5},
RegionFunction -> Function[{x, y}, y <= max],
AspectRatio -> Automatic,
PlotRange -> {{-1, 0}, {0, 1}},
Frame -> True,
Mesh -> True,
PlotLabel -> max
],
{max, {0.7, 0.707}}]
Both lines should be visually the same. The mesh points indicate that the function was repeatedly evaluated far outside the allowed region in the second plot, but nevertheless it didn't try to refine the location of the region boundary.
The problem is very sensitive to the choice of region boundary.
To work around it, you can either change the interval of x in the Plot command to something slightly bigger or smaller than $\pm 5$, or increase the number of PlotPoints as Silvia already pointed out.
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Mathematica by default cannot always guess what humans find aesthetic. In your case, it assumes the standard aspect ratio of 1/GoldenRatio. Specifying a ratio of 1 though is not enough, as the plot ranges are still automatically selected by Mathematica to include most of the data (in this case, all). So to plot a correct shape, you have to give both options explicit values, and the aspect ratio must reflect the ratios of the $x$ and $y$ ranges.
Plot[Table[-x + k, {k, -2, 2, .1}], {x, -5, 5},
PlotRange -> {{-5, 5}, {-2, 2}},
AspectRatio -> 2/5,
RegionFunction -> Function[{x, y}, (x^2 + y^2) < 1]]
answered Jun 7, 2013 at 6:58
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2$\begingroup$ Or, for the lazy:
AspectRatio -> Automatic. $\endgroup$ Commented Jun 7, 2013 at 7:01 -
1$\begingroup$ @0x4A4D True, but I don't find that didactic enough :) $\endgroup$ Commented Jun 7, 2013 at 7:02
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2$\begingroup$ @0x4A4D and Istvan Zachar - I thougth the main problem is existance of those 2 elongated lines, elliptical shape is obvious. On Your
Plotstrange lines are still there but it can be easily fixed withPlotPointslike Sylvia has said. $\endgroup$– KubaCommented Jun 7, 2013 at 7:21 -
$\begingroup$ @Kuba, that's also why I told the OP to tweak the range of the independent variable. Nobody seems to take my suggestions seriously... :P $\endgroup$ Commented Jun 7, 2013 at 7:54
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$\begingroup$ @0x4A4D I think it is because there is no precise question. Some think ellipsis is an issue and others are focusing on those lines. $\endgroup$– KubaCommented Jun 7, 2013 at 8:24
{x, -2, 2}and settingAspectRatio -> Automatic. $\endgroup$RegionFunction. I told you to change the plotting range. You used{x, -5, 5}previously. $\endgroup$PlotPoints. $\endgroup$