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Re: how to handle negative data and position values for log scales

From: Rik
Subject: Re: how to handle negative data and position values for log scales
Date: Mon, 08 Nov 2010 12:17:14 -0800

> Should the change I just pushed be left in place, or shall negative values 
> for different objects be handled differently for each object type for log 
> scale plots?
> For example, what if text objects for a semilogy plot have a negative 
> y-positions? Perhaps my change should remain and something special be done 
> for text objects?
> Ben

I think your patch is fine.  Although it might be clearer to move the code
into each of the sub-objects which is affected, it would also create a lot
of redundancy.  When we next need to tweak this issue we'd find ourselves
hunting through the code to find all the occurrences to update.  Your
current code is concise and represents just one instance to update.

As for negative y-positions, there is currently no support for it at the
axis level.  That would need to be addressed before considering whether
objects can be placed at negative locations.  In my opinion this is too
small a corner case to worry about.  If you're interested though, try
running a semilog plot on Matlab and forcing the y-axis to display negative
values.  What do they do?

Note that confining placement to the visible axis isn't really much of a
restriction.  For example, the left-axis on the plotyy demo goes to -1, but
the right-axis is log scale and starts at 10^-1.  One can still place
objects that seem to occur at negative positions relative to the left-axis
by using the second axis and y-positions that are positive and above 10^-1.


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