Add example images in README.md

Author: mrkn

README.md

@@ -10,7 +10,7 @@ $ gem install unicode_plot
 
 ## Usage
 
-```
+```ruby
 require 'unicode_plot'
 
 x = 0.step(3*Math::PI, by: 3*Math::PI / 30)
@@ -28,12 +28,59 @@ You can get the results below by running the above script:
 
 ## Supported charts
 
-- barplot
-- boxplot
-- densityplot
-- histogram
-- lineplot
-- scatterplot
+### barplot
+
+```
+plot = UnicodePlot.barplot(data: {'foo': 20, 'bar': 50}, title: "Bar")
+plot.render($stdout)
+```
+
+<img src="img/barplot.png" width="50%" />
+
+### boxplot
+
+```
+plot = UnicodePlot.boxplot(data: {foo: [1, 3, 5], bar: [3, 5, 7]}, title: "Box")
+plot.render($stdout)
+```
+
+<img src="img/boxplot.png" width="50%" />
+
+### densityplot
+
+```ruby
+x = Array.new(500) { 20*rand - 10 } + Array.new(500) { 6*rand - 3 }
+y = Array.new(1000) { 30*rand - 10 }
+plot = UnicodePlot.densityplot(x, y, title: "Density")
+plot.render($stdout)
+```
+
+<img src="img/densityplot.png" width="50%" />
+
+### histogram
+
+```ruby
+x = Array.new(100) { rand(10) } + Array.new(100) { rand(30) + 10 }
+plot = UnicodePlot.histogram(x, title: "Histogram")
+plot.render($stdout)
+```
+
+<img src="img/histogram.png" width="50%" />
+
+### lineplot
+
+See Usage section above.
+
+### scatterplot
+
+```ruby
+x = Array.new(50) { rand(20) - 10 }
+y = x.map {|xx| xx*rand(30) - 10 }
+plot = UnicodePlot.scatterplot(x, y, title: "Scatter")
+plot.render($stdout)
+```
+
+<img src="img/scatterplot.png" width="50%" />
 
 ## Acknowledgement