Fix vertical lines for ordinal charts

Charts using ordinal scales on the x axis with .renderVerticalGridLines
set did not work because ordinal scales do not have a .ticks function.
This patch checks for the existence of a .ticks function on the x scale
before trying to use it when rendering vertical lines, and if there isn't
one uses the scale's domain.
add a failing test for #696

for @jcamins (thanks!)
Merge branch 'pr/696'
line length; consistent test wording; artifacts

wrapping up #696