Improve underscore support in LaTeX

The `underscore<https://ctan.org/pkg/underscore>`_ package is now a requirement to improve support for underscores in LaTeX.

* a minimal working LaTeX distribution

* `Graphviz<http://www.graphviz.org/download>`_

* the LaTeX packages *cm-super* and *dvipng* (if your OS bundles TeXLive, the

* thefollowingLaTeX packages (if your OS bundles TeXLive, the

"complete" version of the installer, e.g. "texlive-full" or "texlive-all",

will often automatically include these packages)

will often automatically include these packages):

* `cm-super<https://ctan.org/pkg/cm-super>`_

* `dvipng<https://ctan.org/pkg/dvipng>`_

* `underscore<https://ctan.org/pkg/underscore>`_

Optional, but recommended:

text.get_window_extent())

deftest_textcomp_full():

plt.rcParams["text.latex.preamble"]=r"\usepackage[full]{textcomp}"

deftest_latex_pkg_already_loaded(preamble):

plt.rcParams["text.latex.preamble"]=preamble

fig=plt.figure()

fig.text(.5,.5,"hello, world",usetex=True)

fig.canvas.draw()

deftest_usetex_with_underscore():

plt.rcParams["text.usetex"]=True

df= {"a_b":range(5)[::-1],"c":range(5)}

fig,ax=plt.subplots()

ax.plot("c","a_b",data=df)

ax.legend()

ax.text(0,0,"foo_bar",usetex=True)

plt.draw()

r"\usepackage[papersize=72in, margin=1in]{geometry}",

self.get_custom_preamble(),

r"\makeatother",