Vertical markers aren't generated properly in line chart.

[matmorel]

Here are two lines :

line_1 = [[0, 6], [11, 0]]
line_2 = [[0, 6], [1, 6], [2, 3], [3, 2], [4, 1], [5, 1], [6, 1], [7, 0]]

With those values, we are expecting 12 vertical markers, but it generate 8 markers.
Which is corresponding to the line_2 length.

In lib/gruff/line.rb we can see that @column_count is used to evaluate vertical markers amount :

if (@show_vertical_markers)
  (0..@column_count).each do |column|
    #trace markers
  end
end

In most cases, this is fine, but an option to override this with something like g.vertical_marker_count = 12 would be great.