feature request: support 'symlog' xscale

[andrenarchy]

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Besides linear and log, matplotlib offers the symlog axis scaling (see docs) which comes in handy for plotting negative and positive data in one plot. The following code shows a symlog figure but fig.tikz has a linear axis scaling.

from matplotlib import pyplot
from matplotlib2tikz import save

pyplot.plot([-1, 1e-3, 1], [1, 1, 1], '.')
pyplot.xscale('symlog', linthreshx=1e-3)
save('fig.tikz')
pyplot.show()

I've no idea in how much work this would result. It seems like there is no direct support for symlog axis scalings in pgfplots...