Added additional unit tests

lib/rgviz/data_table.rb

@@ -5,6 +5,7 @@
 require "rgviz/data_table/column_value_filter"
 require "rgviz/data_table/column"
 require "rgviz/data_table/sum_column"
+require "rgviz/data_table/max_column"
 require "rgviz/data_table/order"
 
 module Rgviz

lib/rgviz/data_table/max_column.rb

@@ -1,6 +1,6 @@
 module Rgviz
   module DataTable
-    class MaxColumn
+    class MaxColumn < Rgviz::DataTable::Column
 
       def evaluate(rows)
         max = nil

spec/rgviz/data_table/max_column_spec.rb

@@ -0,0 +1,22 @@
+require "spec_helper"
+
+describe Rgviz::DataTable::MaxColumn do
+
+  before(:each) do
+    @col = Rgviz::DataTable::MaxColumn.new("a", "max(a)")
+  end
+
+  it "finds the max column" do
+    rows = [{"a" => 1}, {"a" => 3}, {"a" => 1}]
+    @col.evaluate(rows).should == 3
+  end
+
+  it "returns nil when no rows given" do
+    @col.evaluate([]).should be_nil
+  end
+
+  it "returns nil when column does not exist" do
+    rows = [{"b" => 1}, {"b" => 3}, {"b" => 1}]
+    @col.evaluate(rows).should be_nil
+  end
+end