Add the way to make background transparent

lib/gruff/base.rb

@@ -367,6 +367,14 @@ def replace_colors(color_list = [])
       @colors = color_list
     end
 
+    # Set whether to make background transparent.
+    #
+    # @param value [Boolean] Specify whether to make background transparent.
+    #
+    def transparent_background=(value)
+      @renderer.transparent_background(@columns, @rows) if value
+    end
+
     # You can set a theme manually. Assign a hash to this method before you
     # send your data.
     #
@@ -378,7 +386,13 @@ def replace_colors(color_list = [])
     #     background_direction: :top_bottom
     #   }
     #
-    # +background_image: 'squirrel.png'+ is also possible.
+    # +background_colors+
+    # - Array<String> format value - background has gradation. (ex. +background_colors: ['black', 'grey']+)
+    # - String value - background has solid color. (ex. +background_colors: 'orange'+)
+    # - nil - background has transparent. (ex. +background_colors: nil+)
+    #
+    # +background_image+:
+    # - Specify the path to image file when it draw the image as background.
     #
     # +background_direction+ accepts one of following parameters.
     # - +:top_bottom+
@@ -396,11 +410,11 @@ def theme=(options)
       reset_themes
 
       defaults = {
-        colors: %w[black white],
+        colors: %w[gray white],
         marker_color: 'white',
         marker_shadow_color: nil,
-        font_color: 'black',
-        background_colors: nil,
+        font_color: 'white',
+        background_colors: 'black',
         background_image: nil
       }
       @theme_options = defaults.merge options

lib/gruff/renderer/renderer.rb

@@ -30,7 +30,11 @@ def background(columns, rows, scale, theme_options)
       when String
         solid_background(columns, rows, theme_options[:background_colors])
       else
-        image_background(scale, *theme_options[:background_image])
+        if theme_options[:background_image]
+          image_background(scale, *theme_options[:background_image])
+        else
+          transparent_background(columns, rows)
+        end
       end
     end
 

lib/gruff/spider.rb

@@ -24,10 +24,6 @@ def initialize(max_value, target_width = 800)
     @max_value = max_value
   end
 
-  def transparent_background=(value)
-    renderer.transparent_background(@columns, @rows) if value
-  end
-
   def hide_text=(value)
     @hide_title = @hide_text = value
   end