Add simple plot spec proposal

Author: darthjee

spec/lib/ruby_gnuplot/plot_spec.rb

@@ -0,0 +1,32 @@
+describe Gnuplot::Plot do
+  subject(:plot) { described_class.new }
+
+  describe '#to_gplot' do
+    context 'when nothing has been set' do
+      it "returns an empty string" do
+        expect(plot.to_gplot).to eq('')
+      end
+    end
+
+    context 'when titles and labels have been set' do
+      let(:expected_string) do
+        [
+          'set title "Array Plot Example"',
+          'set ylabel "x"',
+          'set xlabel "x^2"',
+          ''
+        ].join("\n")
+      end
+
+      before do
+        plot.title  "Array Plot Example"
+        plot.ylabel "x"
+        plot.xlabel "x^2"
+      end
+
+      it "sets title and axis labels" do
+        expect(plot.to_gplot).to eq(expected_string)
+      end
+    end
+  end
+end