Thread: [Matplotlib-users] Getting the set of point actually plotted (from Sage)

Hello all !

I'm sorry if my question is not clear, but I do not know ho to produce a 
simple example.

I'm plotting the graph of an implicit given function (say x^2+y^2=3) 
using Sage.

What I know it that
1. when I ask sage to plot   implicit_plot( f==3,(x,-5,5),(y,-5,5) ),
     Sage computes f-3 on an array of points in the square (-5,-5)x(5,5).
2. Sage creates an object matplotlib.figure.Figure that contains 
somewhere the information about the array of computed points and ask 
matplotlib to plot it

What I understood is that somewhere in matplotlib, the values are parsed 
and a path is created. That path is the set of points on which f-3=0


My aim : catch the set of points that satisfy f-3=0. That has to be 
stored --or at last computed-- somewhere in matplotlib.figure.Figure

I read the source, but I'm really lost.

The final purpose is to know the bounding box of the points that are 
*actually* plotted without taking into account the axes, labels and 
other decorations.

Does someone know how to do that ?

Since the object I have on hand is created by Sage[1] in a quite complex 
way, I'm sorry to not being able to furnish an example.


Thanks
Laurent


If it can help, I have the following in a Sage terminal :

sage: var('x,y')
sage: F=implicit_plot(x**2+y**2==2,(x,-5,5),(y,-5,5),plot_points=100)
sage: F.matplotlib()
<matplotlib.figure.Figure object at 0xbfb60ac>
sage: F.matplotlib().get_children()
[<matplotlib.patches.Rectangle object at 0xc144e4c>, 
<matplotlib.axes.AxesSubplot object at 0xc14472c>]

I really do not understand where is the data ??? In the Rectangle ? In 
the Axes ?

On the Sage's side, the discussion is here :
http://ask.sagemath.org/question/359/get_minmax_data-on-implicit_plot


[1] www.sagemath.org

Le dimanche 06 février 2011 à 14:29 +0100, Laurent a écrit :
> If it can help, I have the following in a Sage terminal :
> 
> sage: var('x,y')
> sage: F=implicit_plot(x**2+y**2==2,(x,-5,5),(y,-5,5),plot_points=100)
> sage: F.matplotlib()
> <matplotlib.figure.Figure object at 0xbfb60ac>
> sage: F.matplotlib().get_children()
> [<matplotlib.patches.Rectangle object at 0xc144e4c>, 
> <matplotlib.axes.AxesSubplot object at 0xc14472c>]
> 
> I really do not understand where is the data ??? In the Rectangle ? In 
> the Axes ?

Your data are embedded in a Line2d object which is itself a child of an
Axes, itself child of the figure. Try:
Fig = F.matplotlib()
ax = Fig.get_axes()[0] # to get the first (and maybe only) subplot
line = ax.get_axes()[0]
xdata = line.get_xdata()
ydata = line.get_ydata()

-- 
Fabrice Silva

> Your data are embedded in a Line2d object which is itself a child of an
> Axes, itself child of the figure. Try:
> Fig = F.matplotlib()
> ax = Fig.get_axes()[0] # to get the first (and maybe only) subplot
> line = ax.get_axes()[0]
> xdata = line.get_xdata()
> ydata = line.get_ydata()

There is something wrong ...

sage: var('x,y') 

(x, y)
sage: F=implicit_plot(x**2+y**2==1,(x,-5,5),(y,-5,5))
sage: Fig = F.matplotlib()
sage: ax = Fig.get_axes()[0] <-- I checked  : it is the only element :)
sage: line = ax.get_axes()[0]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call 
last) 
 

TypeError: 'AxesSubplot' object does not support indexing 
 

sage:line=ax.get_axes() 

sage:type(line) 

<class 'matplotlib.axes.AxesSubplot'>

However, using some "grep get_ydata" from that point, I suceed to track 
my information.
It was in the segments argument of matplotlib.collections.LineCollection

Now I'm tracking back the information ... It is in
mcontour.QuadContourSet(self, *args, **kwargs).allsegs
in the method matplotlib.axes.Axes.contour()

I'm almost done.

Thanks for help !

Have a nice afternoon
Laurent

PS :
I'll post the final answer here :
http://ask.sagemath.org/question/359/get_minmax_data-on-implicit_plot

PPS :
Argh !! Someone already did !