Re: Puzzling aspect of logscale implementation

[Hans-Bernhard B. <HBB...@t-...>]

On 05.11.2010 19:50, Ethan Merritt wrote:
> I have encountered a problem while trying to predict or adjust axis scaling
> in advance of actually issuing a plot command.   The problem is that the
> "set log" command sets the field AXIS.base, but the corresponding field
> log_base is not initialized until a subsequent plot command invokes the
> macro AXIS_INIT2D.   Instead it is set to zero.

It's _initialized_ to zero.  It's set to zero only by the AXIS_INIT*D 
macros themselves, and that only if the axis is not logscaled.

And it would appear to happen so late to allow implementing the 
'islog_override' argument for AXIS_INIT3D.

> Why would you ever want to have AXIS.log_base out of sync with AXIS.base?

Well, that "you" might be me, I guess.  I started axis.[ch] as a 
renovation project.  That was finished by the release of gnuplot 3.8e, 
exactly 10 years ago this week.  But most of that code I just refactored 
from lots of individual arrays (min_array[], log_base_array[], ...) into 
a single array of structs, so the code you're looking at is actually 
even older than that.

OTOH, none of the code using log_base is or ever was meant to be used 
before AXIS_INIT* anyway, so it's not really out of sync.

> Can anyone think of a reason why it isn't correct to always set log_base
> at the same time the base itself is set?  I.e.,  on every call to "set log".

One part of the problem is: what value do you set it to when the axis is 
_not_ logarithmic?  In theory, it would have to be negative infinity --- 
but that can't be done reliably.  Zero is the next best thing.

> And everywhere that it is set to zero should be removed, since zero is
> never a legal value.

No, it shouldn't.  Precisely because it's never legal for a log axis, 
it's just about the perfect signal for "this axis is not logarithmic". 
And since axes are non-logarithmic by default, that's the right initial 
value.