Support symlog scaling option

[markcmiller86]

Is your feature request related to a problem?

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User requests a new kind of logarithmic plot that instead produces signed log outside some interval [-C,+C] and produces linear inside that interval and have C be a variable of the scaling.

This is similar to a feature in MATLAB

[markcmiller86]

Just capturing here how I did this with expressions. The problem with the expression approach is that applying the scaling to different variables being plotted requires editing the expressions.

Would it make sense to allow a plot's scaling feature to be implemented as an expression thereby applying the expression to the plotted variable before it gets plotted. This would make doing different scalings very easy for users and reduce code development burden for developers.