Tick Marks and Intercepts

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Good catch. My guess is that this is a matplotlib thing.

That said, the graphics doc suggests this usage of tick_formatter is not really supported - it was intended for symbolic numbers, not floating point ones, though this is a very nice workaround for formatting. The following works (and doesn't look as nice):

plot(1-4*x^2,.49999,.50001,tick_formatter=[.00001.n(digits=7),None])

It turns out that the (Sage) issue is that floating point numbers in RR are also in SR the symbolic ring, which triggers this problem. But in principle this should still work - the problem is that matplotlib is being passed things that turn into -4e^-5. Note that this doesn't work either

plot(1-4*x^2,.50001,.50002,tick_formatter=.00001.n(digits=7))
ValueError: 
\text{\texttt{{-}8e{-}05}}
^
Unknown symbol: \ttext (at char 0), (line:1, col:1)

so the problem is the addition of the \text when we already have the \texttt there.