Logarithm on base other than 10 for chart axes

[Leyan Ouyang]

I have been looking at what would be needed to support charts using a 
logarithm axis with a base different from 10.

It is already supported by the LogarithmicScaling object and using the 
API it is already possible to generate a chart with a log axis in a 
different base, so it is only lacking the configuration UI and 
read/write files. I have two questions about it:

 1. It seems there is currently no explicit support in ODF, but it is
    coming in ODF 1.4: I saw a discussion regarding this in
    https://issues.oasis-open.org/browse/OFFICE-3936, but I am not sure
    about the expected behavior and the key part has been converted to a
    smiley. Is the idea to write "log(2)" or "0.30102999566398114" for
    the interval-major attribute? The second is what is already possible
    and done today (see for example
    https://askubuntu.com/questions/1105001/libreoffice-x-axis-log2),
    but if we want a nicer UI and offer to configure "2" or "10" or
    whatever, does it mean we need to check if the interval-major value
    looks like the log of an integer?
 2. Regarding Office Open XML, it is supported and LibreOffice does
    retrieve a value mofLogBase in oox, but I did not find how to create
    a LogarithmicScaling with an argument to provide the base (or modify
    it once created). The object is created today with
    "LogarithmicScaling::create(
    comphelper::getProcessComponentContext() );", how can I pass an
    argument on top of it?

Regards,

Leyan
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