change testing code for more automation

Author: v0dro

CONTRIBUTING.md

@@ -26,3 +26,23 @@ When the `draw` method in `Axes` is called, the call sequence is as follows:
 * Consolidate plots like bar plots into 'Multi-' plots.
 * Figure out location of the Axes title.
 * Figure out location of the legends.
+
+## Test infrastructure
+
+Since it is quite tough to generate the exact same plot with the exact same
+pixels on all systems, we perform automated testing by running the same
+plotting code twice, saving the generated files of each run in separate files
+then comparing them both pixel by pixel.
+
+To make this as smooth as possible, we use the `RSpec.configure` method to define
+an `after(:example)` block which will run each example twice, save the image generated
+by each run to a separate file and then compare both the files.
+
+The `after(:example)` block requires a `@figure` instance variable which it will use
+for performing the plotting. A check will be performed for the `@figure` instance
+variable before the example is run.
+
+## Artist defaults convention
+
+Due to nature of a viz library, each Artist tends to have many instance variables
+for storing various kinds of information about the Artist.

lib/rubyplot/artist/figure.rb

@@ -66,16 +66,16 @@ def write file_name
 
       def setup_default_theme
         defaults = {
-          marker_color: 'white',
-          font_color: 'black',
+          marker_color: :white,
+          font_color: :black,
           background_image: nil
         }
         @theme_options = defaults.merge Themes::CLASSIC_WHITE
         @marker_color = @theme_options[:marker_color]
         @font_color = @theme_options[:font_color] || @marker_color
         @backend.set_base_image_gradient(
-          @theme_options[:background_colors][0],
-          @theme_options[:background_colors][1],
+          Rubyplot::Color::COLOR_INDEX[@theme_options[:background_colors][0]],
+          Rubyplot::Color::COLOR_INDEX[@theme_options[:background_colors][1]],
           @width,
           @height,
           @theme_options[:background_direction]