axis_type::range_string conversion breaks small values

[A-Little-Bit-Pseudo-Random]

in axis_type::range_string(), the axis limits are converted to strings using std::to_string.

This internally uses sprintf("%f") which has a limit of 6 decimal places (not 6 significant digits), with no way to modify the precision it uses.

In our application, it's not uncommon to want to plot numbers in a small range (1.0e-07 or less). This results in the min and max ranges being equal (and maybe both zero) in the string, therefore nothing gets plotted. So I'm stuck having to detect that condition and prevent the user from setting the range manually.

It would be much better to use something like sprintf("%1.8g") which uses significant digits, rather than decimal places.

The only other places where I see std::to_string are for the run_empty_plot_command (harmless) font sizes (harmless), origin (possibly a problem), and size ratio (harmless).