Re[2]: [Matplotlib-users] compresson ratio

[Alan G I. <ai...@am...>]

Alan G Isaac <ai...@am...> writes:
>> So this is almost a feature request.  The only problem is
>> that gnuplot's size command does not seem obviously suited
>> to the matplotlib.matlab model, and I'm not sure what a good
>> version would be.  I think it would be something like "get
>> the current axis, keep the "long" axis (as implied by the
>> ratio) full length, and "shrink" the shorter axis as
>> necessary to achieve the right ratio.  This would respect
>> the existing margins to everywhere except by the shrinking
>> axis.  However it would probably be better to adjust the
>> figure size as well, to keep the margins unchanged.


On Mon, 13 Sep 2004, John Hunter apparently wrote:
> I'm not fully understanding you, in part because I find the
> terminology ambiguous. Could you give an example of what you want to
> achieve (eg a scatterplot with a square viewing window).  For
> concreteness, we have
>     ylim / xlim   : the data view limits
>     axh  / axw    : the axes height / width, relative coords 0-1
>     figh / figw   : the figure width and height in inches
>     dpiy / dpix   : the display devices resolution in pixels in the
>                     vertical and horizontal directions
> In order to get true physical sizes and ratios, we need to know dpiy
> and dpix.  matplotlib currently only supports a single dpi, though it
> is still possible to hack true sizes by adjusting for example, figh
> and figw to compensate for variation in dpiy and dpix.


In gnuplot if I want to set a compression ratio
of unity for a figure that has an xrange of [xmin,xmax]
and a yrange of [ymin,ymax], I just say
set size ratio (ymax-ymin)/(xmax-xmin)
So e.g. if I do this when I am plotting a function f:R->R
then the unit size will be physically identical along
each axis.  So for example if I plot a circle it looks
like a circle and not an oval.

In a very clever move, gnuplot also allows
set size ratio -1
to achieve the same effect.

Crudely: I'm looking for something just as simple in
matplotlib.matlab.  My last post outlined what I
think the current procedure is to achieve the same
effect, and it is much more cumbersome.  While this
matters little when one is leisurely writing a script,
it matters a lot when one is doing interactive plotting
in a classroom setting.

Now you point out that this request has an ambiguity:
what "physical" medium am I talking about?  Specifically,
a true compression ratio on screen requires tricky stuff
if dpiy != dpix.  OK, I assume ignoring this will yield
a "close enough" result in a classroom setting, so the
only time I need it to be exact is when I produce figures
for paper display.

Hope that is clearer.

Thank you,
Alan Isaac