Fix spelling mistake in a comment.
Replace TeXBuilder#echo with #begin_chart and #end_chart.

TeXBuilder presents higher-level methods that know the
elements of a chart; #echo was too generic / low level,
and did not fit this pattern.
Retire unused method TeXBuilder#newline.
Rename TeXBuilder#line to #gridline.

TeXBuilder presents higher-level methods that know the
elements of a chart; #line was too generic / low level,
and did not fit this pattern.