Support area charts

[hisham]

I added support for area charts (similar to google analytics charts where the area under the line graphic is filled in).

Here's my git diff:

--- a/lib/gchart.rb
+++ b/lib/gchart.rb
@@ -380,7 +380,14 @@ class Gchart
end

def set_range_marker(options)

• orientation = ['vertical', 'Vertical', 'V', 'v', 'R'].include?(options[:orientation]) ? 'R' : 'r'
• vertical = ['vertical', 'Vertical', 'V', 'v', 'R']
• horizontal = ['horizontal', 'Horizontal', 'H', 'h', 'r']
• area = ['area', 'Area', 'A', 'a', 'B']
• orientation = vertical.include?(options[:orientation]) ? 'R' : options[:orientation]
• orientation = horizontal.include?(options[:orientation]) ? 'r' : options[:orientation]
• orientation = area.include?(options[:orientation]) ? 'B' : options[:orientation]
• orientation = ['R','r','B'].include?(options[:orientation]) ? options[:orientation] : 'r'
  "#{orientation},#{options[:color]},0,#{options[:start_position]},#{options[:stop_position]}#{',1' if options[:overlaid?]}"
  end