Re: [matplotlib-devel] Scale independent drawing

[Eric F. <ef...@ha...>]

Ryan May wrote:
> Hi,
> 
> I'll continue my current flood of emails to the list. :)  I'm finally 
> getting back to my work on Skew-T plots, and I have a semi-working 
> implementation (attached.)  It runs, and as is, plots up some of the 
> grid, with the x-value grid lines skewed 45 degrees to the right (as 
> they should be.)  The problem is as you resize the plot horizontally, 
> some weird things happen.  First, some of the lines that start overlaid 
> end up separating as you expand the plot.  The difference is between 
> what is added using ax.plot and what is added using ax.vlines.  The 
> former adds Line2D objects while the latter adds a LineCollection which 
> is holding path objects.  I'm really not sure what's going on there. I'm 
> not done checking it out yet, but I'm curious if anyone has any ideas 
> off the top of their head.
> 
> The second issue, which is more pressing, is when you resize vertically, 
> the axes limits of the plot don't change (good), but unfortunately the 
> lines don't stay connected to their lower y-coordinate in data space 
> (bad).  I'm really needing to draw things in a coordinate system that's 
> independant of the data scale but also doesn't depend on the aspect 
> ratio of the axes, so that I can get lines (and data plots) where the x 
> gridlines are always at a 45 degree angle and the lower Y-value point 
> stays fixed.  By problem right now is that while I can find the lower 
> left corner in pixel space and use that to do the proper adjustments, 
> this changes when you resize.  This changing is especially important in 
> the y-direction.  What I need is either of:
> 
>     1) Axes space adjusted for aspect ratio (and updated with resizes)
>     2) Pixel space relative to some corner of the axes
> 
> Or something similar that I don't know about.  Any thoughts, or do I 
> just need to come up with some magical combination of transforms that 
> works?  You can see what I have so far in my attached file.
> 

Ryan, based only on your description of the problems, and of what you 
need, I think the answer, or at least part of it, may be in good old 
quiver.  Look at the way the transform is being generated depending on 
the units chosen, and note that preserving a specified angle on the page 
is part of it.  Also note that the transform has to be regenerated on 
resize events, so a custom draw method is required.

(Mike D. translated my original quiver code to use his transforms 
framework.)

It seems like there should be an easier-to-use and more general way to 
do these sorts of things, and maybe there is--or maybe it can be ginned up.

This reminds me of a thread a long time ago regarding adding hooks so 
that classes could register methods to be executed before their artists 
are rendered but after things like window and axes sizes have been 
determined.

Eric