setLogScaleDoubleSi​ded

Converts a plot to log-scale preserving negative x values also shown in log scale. Call backs will keep 2 sides synchronous.

When plotting a spectrum in log scale one normally has to be limited to watching the positive side only.
This function addresses that issue.
% A function to show both positive and negative logarithmic plots, via a second axis,
% with call backs to fix limits and curve's properties symmetrically on the fly (via listeners).
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% A second axis (axH_NEG) is created and its handle stored in user data
% The negative data is shown on this second axis
% After 1st creation a second creation has no effect
% EXTRA features :
% - After 1st creation a second creation has no effect
% - Setting limits for one axis, or zooming, or pan will automatically apply to both axes (listeners)
% - Changing a property of the positive side curves will automatically propagate to the second axis.
% - xlim on one axis with valid limits will atomatically set limits on other side too to keep symmetry
% - call to xlim applies to currently active axis, when positive is active an xlim with neg values will be ignored
% - legend can now be moved around also on negative side
%
% M.Ciacci, 03/12/2020, rev4